Crazy Train Brain

I started dating someone a few months ago and for the most part things have been great! After many dating and relationship disappointments, it seems that he’s someone I can consider a future with – he seems to be a good one. But lately he’s been a little less available to spend time together (he got sick, had to work late, etc…) and my brain wants to go to crazy town. I start thinking thoughts that maybe he’s lying when he says he’s not available, maybe he’s seeing someone else, or becoming less interested without telling me, or isn’t actually serious about me or having a future together.
I want to continue thinking good things about him and our budding relationship but struggle with the fear and paranoia over the small things. How do I not let my thoughts sabotage this? I’m not even sure how to model this. What goes in the C line?

Thank you!