I know you feel all kinds of horrible but what would make you feel better? It sounds like you want to punish yourself for eternity. Your beloved cat did forgive you, and the 5 years he had with your love are more than a huge number of cats and dogs ever get with real love, so you still come out on the plus side of the equation. You gave him a good life. Chances are excellent that if you'd never shown him any hatred he still would have died at exactly the same time and way while you were elsewhere.

If you need to beat yourself up eternally OK. But there's no reason for it. You learned your lessons and both you and your cat are better off for having met. Every cat deserves to be loved happily as you and he grew to love each other. If the cat had only ever known your ex, what a cold relationship that was, that she didn't truly love him either.

You and your cat both came out on top, and you both have a lot to be grateful for. You took him from a really pretty lonely existence with your ex to a happy and fulfilled life being truly loved. That makes up for everything that came before. I understand your self hatred but if you could understand this maybe your self forgiveness could begin.