Overthinking too much

I was wondering if anyone had any key strategies for when it comes to overthinking. Whenever I have sex with my bf, everything is amazing. However, whenever my period is a few days near I always overthink about the situation and I start to believe I am pregnant. This has happened the last few times and I've even delayed my period because of it. I end up going into depression because I start to believe I am pregnant and I'm not a huge fan of children. My bf and I do take precautions, in fact we don't even have sex when I am ovulating and we still use condoms even when I'm not. Is there any way I can stop psyching myself out?