First of all, welcome to TeenHelp.

I understand you really like this guy. It sounds like he makes you feel validated and worthwhile and that's very appealing in a person. Sometimes it's not often that someone makes us feel that way. However, this guy is much older than you AND he has a girlfriend. I know that you like him, but it's wrong to be involved with him beyond a platonic level if he's in a relationship with someone else. I don't think you are "weak" or "bad" for kissing him; if anything, I think he's taking advantage of you. You're young and he knows you like him and he's using that to entice you into a relationship with him. I don't see you as being at fault in this situation. However, the right thing to do would be to set a firm boundary with him and let him know you like him, but you don't want to be more than friends so long as he has a girlfriend (and, depending on where you live, a physical relationship might be illegal between the two of you anyway). As hard as it will be to turn down someone you really like, it's the right thing to do in this situation. What you're doing is not fair to his girlfriend and an she could get hurt as a result of your actions together. So, I suggest being polite, but firm, in telling him where you stand.

Good luck and feel free to PM me if you need anything else.