I've been sitting still for an hour, watching, listening, in the dark. Macaques swing through the cliff-top trees opposite, the drilling of cicadas has begun. Frogs take up the chorus below, the river rushes ahead, gibbons whoop in the distance. Even the leaves are noisy, falling around the deck of my beautiful treehouse with a thud. Stars appear to rush towards me; fireflies twinkling in the Takian trees.

I'm staying at Our Jungle House in Khao Sok – Thailand's largest national park, near Phuket. Run by social entrepreneur Bodhi Garrett, it's a model of ecotourism but doesn't shout about it. What you see is tasteful accommodation, all soft lighting and wood carvings while, in the background, they're quietly building schools and developing community-based tourism projects.

I took a group tour to Cheow Larn lake with Por, a young local guide, and we ventured north, three hours by longtail boat to Klong Saeg Wildlife Sanctuary, leaving the rest of the tourists behind. It's magical and remote, mist drifting over the jutting limestone karsts, the lake banked by thick jungle. We hiked up wild elephant trails, followed bear tracks, explored lakeside caves and ate fish caught beside the bamboo rafthouses we would sleep in. Before bed, Por took us on a night safari where we floated silently past shy gaur – tragically, the heads of these endangered bison can earn poachers £2,000 apiece.

It was all a far cry from where I started the year. January blues were abetted by minor heartbreak and I planned a healing trip to Thailand, Cambodia and Vietnam. So far, so cliched. Friends, recalling youthful travels, promised adventure. Contemplating my more mature 41 years, I privately worried at the prospect of travelling among twenty-somethings headed for full-moon parties. Instead, I booked into a yoga centre on Koh Phangnan. I would emerge serene and calm, I told surprised friends. Arriving at the retreat, I passed men chanting among – or perhaps to – the trees. In a temple, strangers discussed receptivity to love and offered to partner me in tantric workshops. Sitting on the resort's beach, I pondered my situation when the faint strain of bongos made a decision easy. I beat a hasty retreat.

Several hours later, I was on the neighbouring island, enrolling on an open-water dive course at family-run Master Divers. Koh Tao is a diving epicentre and, as a result, the main town, Mae Haad, is also a backpacker's haven, its narrow pathways and rising streets crammed with budget accommodation. A basic, clean beach bungalow at Ananda Villas costs £23 a night (rooms start at £10) and, as with much of Thailand, there's excellent affordable food everywhere. Master Divers are keen environmentalists so, at lunchtimes, I used their reusable plastic containers to fill with fresh papaya salads and spicy noodle soups from nearby street stalls. Mornings were spent in the classroom, afternoons in the sea in small groups, under the expert guidance of Simone, a charming German divemaster.

We got to grips with the basics at one of the most scenic sites, Japanese Gardens. It's ideal for learners and snorkellers, with the reef across the bay going from paddling depth to 14m, vibrant soft and hard corals, and a wide variety of fish: wrasses, parrotfish, titan triggerfish. Once we'd mastered safety techniques, Simone led the way as we glided past lobed pore coral heads fully five metres high, and fields of slipper corals. It wasn't long before we stumbled upon a hungry turtle, chomping away, unperturbed by the captivated audience of divers: you can see why people flock here to learn to dive.

I was smitten, and this newfound passion saw me heading, freshly qualified, to Khao Lak for a liveaboard trip to the Similan islands with an underwater camera. Happy days were spent launching myself into the deep from Sea Dragon's dive boat, photographing turtles, barracuda and stingrays, and using up my air trying to outswim tuna. We were the first divers to visit the East of Eden site, reopened after two years, and here I got up close, too close, to a giant moray eel I later discovered was almost certainly Scarface – a known resident of the table coral at 16m depth, and infamous for having bitten a diver's thumb off. Evenings were passed swapping stories and pictures with other divers over the Thai kitchen crew's fragrant curries.

The only time I felt "older" was on the backpacker bus trail from Koh Tao to Khao Sok, where some studiously jaded youngsters viewed my cheery optimism with the contempt it obviously deserved. "God, Thailand's so slow," they whined, even as the bus company sent a car to reunite the ingrates with a forgotten backpack. Not all youthful backpackers are cast from the same mould. In Koh Tao, I had great fun with a trio of twenty-somethings from Paris, and Khao Sok was memorable for the stunning scenery and wild animals, as well as for sharing those experiences with other travellers. I set off for Phnom Penh, happy I'd made lifelong friends.

But some things are best done alone. My first day in Cambodia was spent in quiet reflection at the killing fields of Choeung Ek. It is Ros Kosal, a survivor of the Khmer Rouge's regime, whose audio narration leads you around the field, past the depressions that mark mass graves. The effects of such recent bloody history are ever-present in a country that's suddenly very young, orphaned and hungry.

However, there is a positivity everywhere, and initiatives you're glad to spend money on. In Siem Reap and Phnom Penh, I visited Romdeng and Marum, training restaurants where the staff – ex-street children – cook and serve fresh, creative food. In Kampot, visually impaired masseurs offer soothing treatments at Seeing Hands, and there's a chilled vibe and decent coffee to enjoy at Epic Arts Cafe, run by deaf and disabled workers. Much of the budget accommodation in Kampot is on Guesthouse Street, where you'll find Pepper Guesthouse offering possibly the finest fish amok in town. This fragrant fish curry, steamed in banana leaves, is Cambodia's national dish.

Kampot pepper stars in this region too, notably at the nearby Kep crab market, where the locals serve it alongside the tasty little critters – well worth a trip. A 15-minute boat ride away, you'll find Koh Tonsay, aka Rabbit Island. Seasoned backpackers who visited Thailand 30 years ago might find the pace familiar: it's pleasingly slow and sleepy, with simple stripped-back bamboo beach huts for just $7 a night, and three hours of electricity daily. I met Lisa here, a Californian backpacker and photographer my own age, and we spent afternoons swimming and walking, evenings lazing in hammocks with cold beers or eating crab (and that fiery pepper sauce) at the quiet restaurants (the two best are part of the family-run bungalow resorts at either end of the beach).

Recharged, I crossed the border to Vietnam, taking a heart-stopping bus ride to Can Tho, the largest city in the Mekong Delta. I hired an automatic motorbike, procured a vague tourist map of the region and set off to explore the Delta. In every direction was river, little tributaries inviting exploration. I scrambled down palm-fringed dirt tracks, the air heavy with the scent of coriander. When patches of drying rice blocked my way I was unsure how to proceed, but smiling women in nón lá paddy hats waved me across, and I rode gently over the grains. It felt wrong, trampling over food on my dirty motorbike, but these piles of rice were ubiquitous and I got used to it.

The delta gradually swelled from tiny backwaters to vast estuaries. I crossed some on ferries so rickety it took all my courage to ride up the wobbly ramps. Commonly, as I rode, children ran alongside, shouting, people thronged, incredulous at this western woman, alone on a motorbike, unable to make even my "xin cha" (hello) understood in their tonal language. I stopped at a streetside wedding party to find myself holding a gorgeous fat baby, posing for photos. Weird and wonderful.

Emboldened, I headed for Ho Chi Minh City. Everyone warned against motorbiking there – estimates put the number of bikes at between two and four million. I told the hotel reception I wanted to rent a bike. "Motorbike taxi?" No, motorbike. "For how many?" Just me. "You? Alone? Out there?"

The rules are a little … loose. People don't stop at red lights. They ride en masse towards you, on what you naively thought was your side of the road. And, yes, they are in their millions, which means you don't need to check your blind spot – there's always someone in it. It was so much fun, so mesmerising, that, come rush hour, I was still riding around, in a thunderstorm, soaked in a summer dress. Proudly, I rolled up to Quan An Ngon restaurant on Pasteur Street, threw my keys down like a local and ordered a delicious banh xeo (sizzling cake). "You obviously live here," a New York visitor said. "Where can I get a good cocktail?"

I had found my place, and my heart was happy.