Help!!!!!! Is forgiveness possible? I feel terrible. (Love question)?

yesturday me and my girlfriend of 18 months and i were talking. she asked did i wnat to kiss her i said yea. then she asked why didn't i and i said that we were talking. she thought that was sweet. later before i was about to leave we kissed and it got a lot intimate. but then she wanted me to leave so i wouldn't get in trouble. but i wanted to keep kissing and asked her not to do this to me. so she kind of went along with it. I feel really bad because i really lost control of myself and didn't listen to her. i was feeling so bad about my self that she could even get thruough to me. i know she has to feel used. i don't want to lose this relationship, but i dont know how i can convince her that i have learned from my mistake. help please?