You ask the difficult questions don't you?
Anyone that says the past is behind you is pretty much delusional in my book.
It has a long reach and sits there in the time gone, reaching forward and determining how you see things even if you don't realise it.
That past is the foundation on which you exist now and it can't be changed. If only it could.
I can't see any of that helping me with the things I can't forgive myself for. They are mental sticking plasters set over a fundamental psychic wound.
I guess it will be different for everyone.
What I try and do, is accept that I am human and flawed. I fucked things up though I did my best. And believe me, my fuckups are serious, causing horrific suffering which i may never be able to forgive myself for.
I can beat myself up about it but it serves no function whatsoever. We all do the best we can in any given circumstance and to dwell in the past is to court pointless self-recrimination.
I don't forget, but neither do I wish to dwell on it. I let it steer my future decisions as best I can, and accept that I am flawed.