Finding out about cheating years later

I just found out that my boyfriend of 2 years cheated about 2 months into the relationship. He was drunk when it happened and it was so long ago that I feel like I can’t be mad about it. He hasn’t done anything since then that would make it hard to trust him, so I don’t know if I’m overreacting by being upset about it. I have no clue how to feel or how to respond to the situation, or if this is something worth ending it over.