Cum during sex

Bree
I can easily cum when i masturbate or use vibrators but I can't cum during sex. Any idea why? I heard once you cum once it'll be easier to cum but that's not the case for me. It's extremely frustrating because I've been with my SO for a year and we AMAZING SEX but I never cum and I've never been able to. What am I doing wrong? Help! 😢