But I can't shake the feeling in my heart saying that I have to stick this out.
Nothing healthy about any relationship is
to stick it out.
The fact that he's already let go of this relationship and you for some (codependent) reason are not letting it go indicates it has nothing at all to do with him or his issues and everything to do with your own (codependency).
There is a very good book out there by Melody Beattie called "Codependence No More" I think you will find it interesting and insightful.