It's important to make peace with people from the past and make amends (not just some phony apology) and see what they taught you. I don't mean you have to approach everyone you used to know, but if you and a friend or therapist feel it safe to approach this character (like say your dad) but don't feel safe approaching another character (like say your ex) write the ex a letter and send it to an appropriate place.

I heard it repeated here to learn from past mistakes, yes. Our brains tend to do this evil thing where we dwell on this mistake and don't realize how well we are doing now. They dwell on harms done to us. We need to let go and move on. Even with PTSD I have been able to move on with help.

Living in the present is possible with the attitude that you can. Though activities and spiritual practices. Some of these mindful activities are nostalgic, as we are creatures living from moment to moment surviving with memories of favorite foods, favorite characters, favorite books, favorite music...etc we become attached to the familiar. Too much of this and we pay high prices on eBay for something we don't truly need and doesn't truly make us happy. Too little and we feel a loss of identity, because all of us grow old and all of us old people need old things to keep us happy and feel young again. Even if it's just a song we haven't heard in a while. I just had that feeling today from a not so old 90s song, nostalgia. It made me feel sad, happy, and it brought back some memory of mom driving me to school on a random day.