The annoyingly direct answer is: do it. Apologize for what you've done. Say it aloud: "I'm sorry for hurting Sue." Then forgive yourself aloud: "I accept my apology for hurting Sue, and I forgive myself for hurting Sue." Then accept forgiveness: "I accept forgiveness for hurting Sue." The conditioned mind will dismiss this as pointless. Try it. From the heart. See what happens.
Here's a graphic I made to illustrate this dynamic:
As you do the work of self-forgiveness, consider these framings. They may help you break your self-punishment cycle & embrace a broader outlook.
Big Bang banging
All phenomena are the Big Bang still banging along. The good, the bad, the ugly, the wtf.
Why cling to negativity about the universe doing its thing, just because you had a 1st person perspective to the events?
What's most important is, you now want to put abusive conduct behind you. That intention can shape the Big Bang's future. Stay with that loving intention, & recognize the negatvity as the ego parasite trying to milk your endocrine system for the drama juice it craves. Stay with your loving intention - I wish I hadn't done that, I want to do better. And recognize that loving intention as an expression of the abundant goodness within you.
2. All is gOd
gOd not only forgives all - gOd celebrates & fully co-participates in all.
gOd sees the full picture, & appreciates how all occurrences, however awful from a human perspective, are all expressions of divine love & flourishing.
Our human minds cannot see this. We see a few grains of sand, compared to the light years of beach available to the mind of gOd.
What gOd did as you then, occurred for an exquisitely perfect reason you'll only truly appreciate when you return to gOd.
For now, focus on the fact that gOd-as-you so badly wants to leave those ways behind & explore a new direction. Honor this beautiful flowering within you, and let gOd's perfect past be gOd's perfect past.
3. You want to be better
Our egos drag us into the imagined future & the reimagined past, trying to yo-yo our sense of self worth between arrogance (nothing's good enough for me) & shame (I'm not good enough for anything).
We exist in the now, a series of present moments.
Now, you want to be better than you've been.
Now, you want to put the past behind & awaken to a brighter future.
Honestly wanting to be better - it's one thing no one can make you do. Either you want it, or you don't. When you don't, you languish in the same loops as always. When you do sincerely want to grow, it's a gift.
You have that gift. Stay with it - the desire, the thoughts, the feelings, the vision (however hazy) of what's possible.
Recognize all the judginess, doom, ugh why bother, speculation, etc., as ego trying to drag you back into its lair.
You are your savior. Your method is not dramatic heroic glory. It's more like the tortoise from Aesop's fable. Point your beak the right way, and step. Repeat. The ego-hare will race around, try to lure you into rushing or quitting. Feel your heart, and let your pulse set your pace.
Your moral compass is already calibrated to healing - your desire to forgive yourself shows this. One day, one breath, one heartbeat at a time, may you be the steady friend & guide you're seeking.