My boyfriend admitted to me that when he was drunk he kissed his ex girlfriend at a party

My boyfriend (23M) and i (22F) have been together for 3 years now and everything was going great until one day he told me that when he got very drunk at a party he kissed his ex girlfriend (they have been broken up for 6 years now). He told me that he regrets it but I can’t seem to get over it and I keep thinkin what else he has done when he was drunk, how can I trust him again after this?