Too much stress

Hello, I am a teenager and only recently have began being sexually active. However, anytime I engage in any sexual activity (not even just sex, which I've only done once) I ALWAYS stress out. Like "oh god, what if I'm pregnant? What if somehow we messed up? I'm feeling nauseus, oh no I'm pregnant." I'm extremely paranoid and it's so mentally draining. I get like this even though I haven't missed a period. (I've never been late, but I'm not sure what to expect with my wacky cycle) I'm not on birth control at the moment, but I'm trying to convince my mother to let me go on it while still being allowed to go to my boyfriend's. If there is literally any way to calm down this paranoia and anxiety, PLEASE give me some tips, I'm desperate.

Thank you.