I'm exactly the same as you. I think my friends think I'm more open than I am - I tell them select pieces of info that don't really matter to me, but never talk about feelings etc. I don't mean to be like that and I'm always people's shoulder to cry on, they'd be fine if I got upset or something, but I just can't physically be emotional or open or anything with people.
I haven't changed really. When I got my first proper boyfriend when I was 15 he eventually got me sharing stuff with him and I finally had an emotional closeness I'd never had before, which was great. I've had several good relationships since, but the problem is I now find it's ONLY with boyfriends that I show the 'real me' almost...I guess that might be true with a lot of girls to an extent but there is so much I hold back from almost everyone. There is a major problem with this: break ups are incredibly difficult to deal with, as it feels like I've lost the only person who knows the important stuff. I desperately try to stay friends with my exes, and as a result have had a few slightly dysfunctional 'friendships' which I would not recommmend. However, I do have exes who are very close to me now and who I can talk to still.
I think you'll either struggle to have a close relationship or turn into me. Both aren't great. I think maybe you should try to open up just a tiny bit more. That's what I'm trying to do. You don't have to tell you friends every detail of your life, you don't have to lean on them, but just try to share a bit more of yourself. I think it does pay off, if you have decent friends to start off with they'll be even better ones.