My girlfriend admitted that her ex fucked her better and it’s getting in my head

I (19M) asked my girlfriend (22F) to be honest about who made her feel the best and she mentioned one of her ex. I was floored but didn’t show it on my face. Now when we have sex it’s all I can think about. I’m fucking her harder and I can’t get off anymore because it’s like what’s the point of me doing it if she’s had better and won’t get that from me. Yes I’m internalizing it. She’s noticed the change and asked me to slow down but I can’t. I can’t get it out of my head.