You don’t feel good about yourself around him. A partner who challenges you can be a good thing, but there’s a difference between feeling inspired to be a better you, and feeling inadequate, and that can be hard to spot at first. But if the guy is making you feel insecure, get out now.
You don’t see a future with him. You don’t have to be ready to tie the knot this moment, but if you honestly can’t ever picture yourself with him in the long-term, you need to end it (or at least make sure you’re both on the same page about the expiration date). Otherwise, you’re holding yourself back and probably setting things up to hurt him.
You think about other guys, a lot. An occasional crush or drool session over another guy isn’t necessarily a bad sign. But if you’re constantly day-dreaming about what your life would be like with other men, you might not be getting what you need from this guy.