What should I do ?!?!

So this might be kind of long but my SO and I have been together for a little over 3 years and I've always been on birth control so we rarely ever use a condom and that's never been a problem since I was his first . I kept having discomfort during sex and bad cramps so I went to the doctor and I tested positive for chlamydia. I want to believe he didn't cheat when he says it but I think I'm just in denial. I still love him but I'm not sure if I wanna be with him anymore. Should I believe him or should I leave him?? I need an outsiders opinion and please don't judge I never thought this would happen 😩