I just got back in touch with a friend who was an asshole in the past. Idk. I think the feeling of cringe when you look back is enough to know you really did change.

You absolutely can forgive yourself. A good way to think of it is how would you forgive a friend if they actually took the steps to change? It may take a while yes but eventually if they work to earn your trust and show that they care then you eventually accept them back into your life and move on. It's the same thing you have to do for yourself. Recognize you said things in the past, make steps to ensure you won't say them again and be aware of how you respond now versus how you would respond in the past, accept that you have changed and move on. No one in this world is entirely good nor entirely evil. We are three dimensional people with hopes, dreams and aspersions. As long as you have forgiven, accepted, and moved on change is absolutely possible.