Why do I have issues?

Ever since I was really little, I remember having sexual thoughts, I remember acting out thingd with barbies that kids my age shouldnt... Is it normal to be so fixated on sexual stuff at a young age, (4 or 5, maybe younger) or do you think something might have happened that caused it? I don't remember anything happening, but most kids I'm around are nothing like I was at that age. I've always been withdrawn, It's hard for me to talk to strangers, especially guys. I've only had like, 2 boyfriends, I've never kissed them though. We didn't last long, probably because we didnt act any different than when we were just friends! I feel weird holding hands, and I hate hugs. I WANT to be in a relationship and fall in love and be able to be comfortable and cuddle and everything, but I just can't and I dont know why! (I'm almost 18 now) Do you think this could be caused by something that happened when I was little and it's affecting me now, or do you think it's just how I am naturally? Please give any advice you have on how to get past being uncomfortable around guys!