Sounds as though Kane your prides been hurt and you are distrusting your girlfriend.
If she is still your girlfriend is there a reason other than you mentioned that keeps this on your mind this many months later? You need to feel that you are a worthy boyfriend for her, and jealousy should not be an issue. I hate to think at your seemingly young age that a gf/bf relationship would be a base for hurting yourself like you did. This was hardly a rejection by your girlfriend, but maybe her insight to a flaming jealousy that you seem to own without real reason. Jealousy in itself is enough to drive your girlfriend away, regardless or without the attention of this other guy.
You are a whole person Kane, and it does not take another person to make you a good guy...relax and let it go. Just MHO. hugs, bj