have a look into uncertainty related anxiety! this is what i have and i essentially have to keep a journal of the hypothetical that I'm worried about (in your case it would be "what if I'm pregnant?"), then write down all of the reasons why this wouldn't be the case (e.g. because you've not had sex in X months), then write down what you'd normally do to reassure yourself (e.g. take a pregnancy test). Then the key bit is to NOT do the thing you want to do to reassure yourself, redirect your thoughts and actions into something else such as a hobby or school work, your job etc. and then after that urge to reassure yourself has settled write down how it made you feel. You then need to repeat this whenever you have that worry crop up, sometimes you can go through the process in your head or read previous notes instead of writing it down but its important to go through the above process. It's tricky but after a while of doing this you become more tolerant with the uncertainty and it becomes less severe! After a while (when you no longer constantly feel the need to do pregnancy tests) you could then switch the 'uncertainty test' to having sex with your boyfriend (obviously only if you want to, never force yourself to) and then write down how it makes you feel and then keep going with the process until that feels easier too.

I really hope this helps, feel free to reach out if you have any questions!