I need to calm down

So I’m starting to over think and panic. I got tested over winter break and was completely clean. I had protected sex with a guy and I gave him oral sex. We’ve talked ever since every day all day he is so nice. He goes to the university where I’m from. My doctor put me on an Sertraline (Zoloft) to calm my nerves and it did keep me from thinking to much about it, but for a week I haven’t taken my medication because well I can’t afford it since I bought my books. Tonight I am sitting in my head thinking of all the symptoms I feel and thinking maybe he gave me and STD and I’m comfortable enough with him to ask and he told me he’s completely clean and when I come home for break we can go together to get checked and didn’t forget to make a side remark “you can even hold the cup when I pee and I’ll do the same for you” I wish I could stop always thinking I have an STD even with the doctor telling me I am fine and that I’m clean and healthy I still was like maybe I should get a second opinion maybe they messed up the charts and labs. I want help I want to stop feeling like this. I don’t want to ruin a perfectly good relationship because of my own mind and worries. He’s so nice and we talk to each other about everything but this thought in my head over powers me and I can’t stop thinking about. I feel like I’m going crazy and I don’t want to spiral back into the mind set I was in six months ago, where I cut all of my hair off and contemplated killing myself all because of the thought I might have something and the sad part is I wasn’t even really having sex I had a boyfriend at the time and we would have sex one time a month and I was constantly stopping to make sure the condom was still on. Me doing that ruined that relationship and I don’t want to ruin this one.