Do I just let go of trying to understand him?
no. but invest in trying to understand where this relationship is at, where its likely to go, what it will take to rebuild or walk away, and where your heart is in all of it.
if youre done, be done; make that hard choice, do the hard things it will take to Detach. dont leave it to him.
multiple make up/breakup cycles, over time, do more and more damage to a relationship. its not going to get better, only worse, without radically changing something.