How to relax yourself

I heard in places during sex it's so relaxing but why do I tense up, I trying to relax and cum with my bf, but can't seem to relax and let it happen naturally, but instead I get tense always before him too I was tense. And I would squirt and cum, but seems like sometimes I can't enjoy my self because of me being tense. How can I relax? If someone understands what I mean