Feeling guilt is one of the biggest destroyer of your mind, body and spirit.
Despite everything that could had happened, should have happened, might have happened DID NOT happen. No amount of self hatred or torment can change anything and trust me you cannot function without coming to terms with it somehow, some way.
I have just went through doing this about my marriage. I now know that no matter what I did to change she would change -nothing-. i changed and changed and she never would. It is out of my control and I can't control her. You could not control Kate, bud.
I won't say let it go because you know what you must do. Message me anytime if you wish to talk.
This song helped me with past guilt and forgiveness of myself and others. It's long but healing.