Basically, my girlfriend and I have been together for almost a year and a half. I havn't been the best of boyfriends in many senses before but I recently got on anti-depressants, and I feel better about a lot of things... But recently my girlfriend started feeling like she misses the single life, and she doesn't want to leave me because she does want to spend her life with me she says, but we made a deal that if she finds a chance to flirt with another guy or kiss another guy as long as it doesn't go too far, she can... She said she wanted the same for me for other reasons, but it doesn't feel right to me, I know that I want to remain hers, and if this is what it takes to do it, then so be it. In a way though it seems like a break-up, because she would never kiss another unless this was in place of she left me... I losing a lot of sleep over this, i'm fine in the day for the most part, but this past week it's been driving me crazy... I just need some kind of advise, I can't tell her that I don't want her to because after all of the time that I have trapped her in this relationship, it was only a matter of time this happened... But I just need some kind of idea of anything to help this... Rather it be for me or for her.
Some other problems
I think you need to go to her and be completely honest. Tell her how you've been feeling. Things like these are hard to dodge and matters really depend on the personalities of the two people involved in the relationship, history, etc.
I'm assuming you're a young couple. A lot of young couples go through things like this because one or the other or maybe even both need to "sow their oats" as they say.