Amelia,

You asked how do you let go as even imagining the post-breakup scenario is hard. Gosh I was just there myself and then I read a beautiful article that helped me make a decision and decide to not remain in that relation despite very strong feelings and attachment (If I can find that article again online, I’ll post it here).

See it like this. You are justifiably not happy with the current scenario, but there ‘was’ a nice time which makes you think that there is potential. You are holding on, expecting and hoping for a change and a happier future. Long-term Change in his behavior and his treatment towards you. Now, no one can 100% predict or guarantee it. Being even extremely optimistic, there is a 50% chance that he’ll change but 50% that he will not change , equal chances (though I think the probability of no change is rather higher as its already the reality, probability of change is banking on potential, reality > potential). Now without a doubt, you’ll be elated if he changes but also ask yourself that if he does not change, is it something that you will be able to live with in this relationship? If you stay, or marry, you have to go in accepting both the possibilities.
That is what made me choose to not remain in the relationship. I was like, all this while I am thinking and hoping it’ll be so nice if he changes but what if he doesn’t? Will I be able to stay happy if things remain as they are? Will I be able to accept it, his angry outbursts and snapping at me? And given that my ex had similar habits of getting angry, shouting and being irritable like your current partner, my answer was no, I’ll be miserable if he doesn’t change. And though extremely difficult, that realisation helps you see through post-breakup times.
I agree with Rose of Yellow that you can’t change him. At max you can inspire him to change but whether he actually puts in efforts to change and changes sustainably, is totally up to him. Ask yourself if that is a risk that you’re willing to take. I also agree with Anita, Specially about the part that sorry means nothing until its backed up with actions and not just short term actions, but long sustainable ones.

That’s just my two cents strictly from my own little experience as your story resonated with mine, I’m not very experienced, so take it with a pinch of salt.

Love,

Jenny