When you try to decide between being miserable with him and being miserable without him

But at the end of the day you know that if you leave, you will eventually feel better, and on top of that, there are chances that in the future you'll find a good partner. There is opportunity for improvement in several aspects.

If you stay, it's guaranteed you'll keep feeling miserable. Even if things "change" temporarily. Remember that they have already showed you that they're incapable of change and self reflection. By going back, you're agreeing to go back to the same situation that has already hurt you before. Your sanity and health will be relying again on an unstable, oftentimes unpredictable person.