Cheating

So I have been with my boyfriend for 11 months and things have gone really well, I was very excited for us to celebrate our year soon. However I just found out that at the beginning of our relationship he slept with another girl and cheated on me. Now I already knew that he had kissed this girl that night as he told me it happened but he has lied for 10 months about having sex with her. I don't really know what to do because it was so long ago but it hurts so much. Does anyone have any advice?