Maybe look into Automatic Negative Thoughts (ANTs) and intrusive thoughts. As you say it can make you feel crazy because it's like a faucet of negativity you can't turn off.

I would look into Cognitive Behavioral Therapy to help tone those thoughts down a bit. If this is a co-dependency thing then maybe look into self-care, too. You should be able to enjoy your free time without him, and without thinking about him. What's the use of alone time if other people are occupying your thoughts? It puts a lot of stress on you, and no one deserves that!

How long have you been dating/married? As other people have mentioned, spending time together 24/7 isn't healthy for any relationship. Again, try to focus on yourself and not worry about him.