I have always thought that, contrary to the presently fashionable blame culture, to a greater or lesser extent our lives are a product of our choices. And while it is usually hard to pin down the exact instant when we took this road or that one, it is not always so. Sometimes we can remember precisely when we altered direction and, in my life, a Damascene moment occurred when I was just beginning what we did not then call my gap year. I had left school in December 1966 and I was waiting to go up to Magdalene College, Cambridge, to study English literature.

My childhood was a happy one, spent in a tall house in South Kensington and later in East Sussex, but my early and mid teens were less successful. Among my contemporaries in the county I did not shine, for I suffered from a bad case of sibling rivalry. I was not in competition with my eldest brothers, Nicholas and David, who were far above me in age, 12 and six years older respectively. Their lives placed them in a different sphere; not only because they had girlfriends and cars, but Nick was a young officer in the Blues, dancing his way round London's ballrooms, and David was just wild. Once he hid out in the shadier parts of Paris for three months until my father agreed to let him skip university. Eventually he turned up working the lights in a revue bar in Piccadilly, which he never confessed to my parents.

On one memorable evening when I was 10 he took me there, having agreed to be my babysitter. The girls, pretty and charming in their sequins and feathers, spent the whole night venting their frustrated mothering instincts on me until David drove me home. I never betrayed him.

This stuff seemed exciting and even admirable, but it had little relevance to my own struggles. My problem concerned my third brother, Rory, a figure of immense beauty and glamour who kept me, plain and ungifted as I was, permanently in the shade. Of course many people have to contend with a prettier sibling, but not everyone has a brother who is invited to model on the front of teenage magazines and is put under a recording contract. On one Valentine's Day at school, he received 47 cards. I got one. From my mother.

Release came from an unexpected source. My mother's sister, Phyllis, had recently been widowed in slightly dramatic circumstances. She had married an Anglo-Argentine in the 1930s and spent her life in South America, ending up in Colombia. The sudden death of her husband had left her in something of a crisis and she decided to open their ranch (or finca as it is known down there) as a children's summer camp, with the help of her sons. But she needed some more unpaid labour and she rang my mother to see if she could borrow a child. Mummy explained that I was the only one available and so the plan was made.

When my father refused to stump up the airfare for what he considered an idiotic scheme, my mother went to the Shell building (where he worked) and somehow blagged me aboard an oil tanker. On every tanker, it seems, there is a commodious apartment, usually empty, known as the Owner's Cabin, at the disposal of the company. The journey would take three weeks, with no sight of land. I would be met in Cartagena and travel down to Bogotá.

The crew was Dutch. The Dutch are a courteous race as a whole and, of course, wonderfully gifted when it comes to languages for the simple reason that they know they must speak some language other than their own if they are ever to enjoy travelling. But they're also a hard-working lot and not much given to frivolity. The net result of which was that I was almost entirely alone for 21 days.

I'd meet the captain and his officers at lunch and dinner, and we would converse pleasantly enough, but apart from that I spent my time in my handsome apartment. This included my own private deck and, as I watched the Atlantic slip past, I got down to the required books for my approaching course. I read Sense and Sensibility, and Hard Times and A Sentimental Journey and Daniel Deronda and Gulliver's Travels, and The History of Henry Esmond… until the letters started to dance before my eyes and I realised I could read no more. So I thought. I thought of my failures, social and romantic; I thought of how much I had to say within the family and how little outside it; of how unsatisfactory my life was and of how much I would like it to improve. And I was not humble in my humiliation. I felt I merited better.

And then it dawned on me that I was travelling to a land where no one knew me. My aunt did not much visit England and we had not met since I was a small child. Ditto her two elder sons, while the younger two had never met me at all. I was going among close relations as a stranger. And in a sudden flash which I can recall even now, I realised this was a unique opportunity to alter my personality – or its public presentation – overnight. I had climbed the gangplank as a shy and insecure bore, an awkward, spotty teenager, only to be danced with as the partner of last resort. But I resolved to walk down it an outgoing, gregarious, confident young man, up for anything, eager for adventure. And that is what I did.

The camp was run by my aunt, her younger sons, Gordon and Nicky, Gordon's girlfriend Anna, and a rather beautiful American called Alison Starkey, all of whom were deceived into thinking I was naturally spirited, jovial and, although it now sounds vain to say it, fun. Thankfully I could swim and ride and knew the rules of the various games, and if I was initially nervous in my masquerade, I was helped by the fact that it was all pretty chaotic.

None of us had the faintest idea how to get the children to go to bed, so they used to stay up all night dancing and only flag out in the small hours. We even had an earthquake and we all ran out of the heaving, jumping buildings to watch the water dance on the surface of the pool. No wonder they begged their amazed parents, who had expected to find them aching to come home, if they could be allowed to stay an extra week.

One afternoon I suggested a scavenger hunt. For those who are too young to know, this was a party favourite in the England of the 20s and 30s and enjoyed a revival in my own day. The guests would separate into small groups, they would be given a list of objects – a policeman's helmet, a Ritz ashtray, an autographed programme from Covent Garden and so on (or the rural equivalent) – and they would set off.

The children at the camp were mainly either from American service families or embassy kids, and certainly none had ever played it before, but they were very enthusiastic. The trouble came when we put "a spider" as one of the desired items and a group of nine-year-olds returned with an enormous tarantula pushing its hairy legs against the glass of the jar they held it in. We explained that it was dangerous and could probably have killed any one of them, but this only prompted the leader, who surely had a future in diplomacy, to ask for extra points.

At last the summer was over and I set off for Cartagena to wait for the tanker that would take me back. My aunt, anxious that I should sleep late, taught me how to order breakfast in bed. Dos huevos fritos con tocino, y tostadas con mantequilla y mermelada, y café con leche para uno, por favor. It remains my only Spanish to this day.

But the point is, it worked. I went up to Cambridge and, the following year, dove into the London Season and I never turned back into the lonely, tongue-tied pumpkin. I did not much cultivate my friends in Sussex on my return, not because I disliked them but because I was no longer the person they had known and it seemed unnecessary to convince them of the new me when there were so many other fish to fry. The rest, if not history, has at least proved more interesting than it might have been. I suppose it is further proof, if proof is needed, that contemplation, the time to think properly, is an essential ingredient to any rounded existence.

Julian fellowes is an actor, director and Oscar-winning screenwriter. His latest work, Downton Abbey, is on ITV at 9pm on Sundays