Sometimes people make mistakes. Why was she with her ex? Was there a reason why they were in a situation where they were alone together? I think by her telling you it shows remorse and it sounds to me that she really was sorry for doing it. You can look past this and give her a chance and if she were to make another mistake then you will have your answers. You have to follow your heart here and if you feel like you want to move forward with her then do that but just be cautious going forward. She broke that trust and now has to work on rebuilding it. She needs to not see her ex anymore if she does get that second chance with you. You have to think, if you made that mistake, would you want her to give you another chance. If the answer is yes, then you need to try and let it go. Good luck.
It sounds like you are both very young. age has a lot to do wirh how one acts. luck jo
Sounds like she messed up, but she realises she did, and she is genuinely sorry and regretful. She loves you and wants to make this work, and it sounds like you love her and want to be with her too. I think, I hope, you have what it takes to get past what has happened.
I truly understand how much this must hurt you. It won't disappear right away, but if you two stick together and she shows you that she loves you and is devoted only to you, then hopefully the hurt will go away after a while. Try to focus on the future, plan fun things for you and your girlfriend to do together, and the past will gradually fade away.
If you do decide to stick with her, try not to keep bringing this up. I know that it will be on your mind a lot, but if you keep talking about it all the time you may well drive a wedge between you, because it will start to feel like your relationship is all about what happened with her ex, rather than about moving forwards together and having fun.
If this took place after 6 years, I would raise an eyebrow. Your relationship is only, only 6 months old. Not old enough for serious committment type stuff. You are still learning about each other. You just dont smooch on an ex, even in the spur of the moment unless some kind of attraction is there. Sorry! You need to slow it down and take it one day at a time and c what happens. It does not sound like she is in love like you are. Nor should she be. Your moving too fast will ruin things as well as a kiss from an ex.
Although this kiss has bothered you I would not let it end the relationship, it was with an ex and maybe just a relapse to old times, if it had been with a stranger or someone else, then maybe it was something to worry about, also it often takes longer than one thinks to get over an old relationship, this may have been part of the process, my ex girlfriend used to get calls from her old boyfriend, she never answered and didn't want to have anything to do with this guy but after so long he still called and hadn't yet moved on. I would have a civil chat to resolve this issue with your girlfriend try to find out why it happened, make sure you don't get mad and say anything you'll regret.