Sexuality issue?! Help.?

Okay so i'm very confused about my sexuality. I'm nearly 20 years old and have been with men all my life but when I was younger, around 13, I kissed a girl and had a relationship with her but I can't really remember my actions for this. When I look back I feel really embarrassed but no one knew about it. I keep having sexual thoughts about women and I've watched loads of lesbian tv shows and movies. I honestly don't know my sexuality and when people ask I say straight. I get really nervous around women and I instantly know when someone is gay, I think they can tell I've been with a girl before too and that puts me on edge. All my friends are straight, my mum and dad are strong Catholics and I do want to get married and have children someday with a man. I think of women and think how nice I can be then all these thoughts go off in my head. I'm scared to experiement because I feel if I go down that road I might not be able to get off it and it will just confirm a worry what I have, that I am infact a lesbian. Can someone help? What should I do?