Boyfriend Lied

We’ve been together almost 3 years. My bf admitted to me a while ago that in the beginning when him and I started seeing each other him and a girl kissed. When he said this I asked him if it was before him and I did stuff (cause to me him and I were official during then) according to him he didn’t think we were official and told me EVEN swore to god that the kiss happened before him and I did stuff but before answering he had to look up in the air for like 5 mins trying to figure out what to say (BS). Now today we ended up on the topic and he basically said the kiss with the girl happened after him and I had done stuff. I’m hurt. Apart of me wants to push it aside because it was so long ago but the fact that he recently lied and even swore to god about it kills me. He pretended to not know when it had happened. So I left his house because I’m so upset and don’t know what to do. Advice?