I think when we focus on past mistakes we only remember the action that we did that caused that mistakes, we don't really ruminate on the possibilities of other actions that we could've taken, would it still led to this life that we have now, or maybe later. Past mistakes are there so we can learn from them. Everyone has past mistakes, some pretty shitty, some that feel shitty but are small.
One of my past mistakes was not graduating high school and dropping out. I didn't get to walk on stage and make my mom happy, but to put on perspective, would she be happy? Life experiences would say no because even though I made a name for myself and got a college degree, she was never happy. Ok, another perspective was that I had to do a lot of things the hard way. Rejected applications, got into my career late, etc., but what's done is done. It doesn't affect me now unless I let it affect me. I would get nothing done if I thought about my past. I try to move forward as much as I can. What is in your past is there, it can't hurt you as worse as it may did before. My question to you OP, is what are you doing in the present? When you're not ruminating about the past, what can you do to make that mistake stay in the past instead of being in the present.