he cheated on me with his ex.

🐬✨
Ok so me and my boyfriend have been dating since last summer until now. Back in the beginning of this year we had a bit of an argument and we weren't talking for a few days. But then he took me out on a date at the end week and talked about everything. Pretty much talking our problems out so we can move forward. 
But, He just told me today that during that week we weren't talking, he saw his ex in public then went back to her house and they had sex. Idk how to deal with the situation. Like I'm sad and disappointed in him but I also want to mush tf out of his ex. But what should I do about our relationship?