Why can't I cum?

Looking for an explanation to why I can't have an orgasm with a guy. People have told me before that it's because I take SSRI's but I don't want to believe that. I don't have trouble orgasming on my own with a toy but I want to experience an orgasm with a guy. Any tips or advice? Does this happen to anyone else? (23 years old and have had 10-20 partners)