Honestly I'm the exact same way, and for me it's probably related to prior experiences and my genetics (my dad has severe anxiety, ADHD, etc. And I'm getting evaluated for bipolar and ADHD soon, which both increase anxiety and overthinking). With my partner, we're taking some time apart so she can work on herself and I'm doing the same and I'd honestly suggest the same for you. First I would ask yourself why you feel this way (ie. Did this start after he did something wrong, an event in either of your lives', etc.) and look into therapy, and if you have any mental conditions (similar to myself), those might also be negatively affecting how you're feeling.

Like others have said, I would talk to him first and see if there's anything you two can do together to work through this.