OK, first of all -
You're quite young, and assuming that you don't have a thing for older women, I imagine the ladies you meet are pretty young too, so perhaps you're just coming on a bit strong with the ladies.
The reality is, that you're really not likely to have found 'the one' at such a young age, and this may be scaring the ladies away a little bit. Contrary to popular belief, women in Britain aren't all obsessed with getting married and having babies.....most of us have ambition, want to travel the world and see everything and do everything before we start settling down, so when guys we're dating start saying 'I love you' etc too early on in a relationship, we're likely to run a mile.
I personally think being a nice guy is more important than getting laid or finding 'love' in your early twenties.....it will have its reward later in life, once women your age have become somewhat jaded by their experiences of the men they cavorted with in their late teens early twenties.
Unfortunately, we women tend to have a head start on you guys......from before puberty we become interested in the feminine trappings.....we learn how to attract men, and often practise our skills on the ones whom we trust not to hurt us, or take advantage. Guys on the other hand, pretty much only notice football until their late teens when all of a sudden it seems like everyone's having sex but them, and they have no idea how to get any action. The thing is, that what women don't learn when they're learning how to attract men, is what kind of man they actually want to attract......that lesson doesn't come, often until they're in their thirties - oh and of course that's when we hit our sexual peak too, of course.
As I've said to so many lovely men online, use the close brother/sister friendships you have to learn about women.....what makes them tick, what they really want out of life and relationships, listen to what they have to say about the men they fancy and those with whom relationships have failed, and hone your flirting skills so that when a woman comes along who's just right, you'll know just what it takes to woo her, win her and make the relationship work.
It's the worst thing in the world to hear, but all you can do is be patient, fill your life with good friendships and interesting experiences, and when you're not even thinking about finding love, it'll happen to you. I promise. x