How to get over the fact that my bf had sex with another girl before me?

ok so before me my guy dated this girl I really hate before me.. but he jst wanted to get in her pants and he dated other girls while he was dating her.. 2 months ago he told me he never got in her pants, and yesterday he told me that he did but that after that he dumped her and its when he started dating me.. he said that im the only girl he has taken seriously since the beginning and he says that he loves me and wants to be with me forever. and we do have sex but now im not gonna be comfortable having sex with him now that I know that he f***** that chick.. how do I get over it?? :'(