I have been in a relationship for almost a year and recently found out that my girlfriend kissed another guy at a party I was not attending. She told me this casually a few days ago and when I lost my temper about this, she said I was blowing things out of proportion and that she wasn’t cheating on me. I don’t know how to deal with this because she frames it as just a kiss, but I would never do this to her and don’t know if she would be okay with me if I were to tell her something similar. If I let it go, I am afraid she may do this again at some point. I am frustrated and considering breaking up with her. Am I being too hasty or over-reacting to a minor incident? I love her a lot, but this feels like disrespect, so I am confused.
You and your girlfriend set boundaries for what this relationship is meant to be like. If she has crossed them, you are entitled to feeling betrayed irrespective of how she justifies it. If she says you are blowing things out of proportion, it is a form of gaslighting because you are being told to stop feeling hurt. Her actions have caused you pain and the only way for her to fix this is to apologise and start by acknowledging that she did something wrong. Without that, you will be unable to move forward because you’re right about this being an act of disrespect.

My girlfriend is definitely not over her ex-boyfriend and is okay with admitting it. She wants me to be patient and says it will happen. What should I do?
She is being honest, but this means your relationship will not be as steady as you like it to be. If you are willing to be with someone who says she isn’t in love with you, that is your prerogative, but you have to do this with your eyes open, knowing the risks involved.