How do I learn to forget the past?

First of all, let me say thank you for writing this question, I know it's not always easy to speak up and express our emotions, feelings, thoughts and concerns with others, so I truly appreciate the effort you are doing by talking about it all and sending this question. Besides, I think it's a very interesting and meaningful topic as we all have lived through situations in the past that we have been carrying for more time than we would like to. Sometimes these situations happen due to our own responsibility, some other times due to another person's responsibility ... In any case, we need to find the path to be able to move forward and live our lives without feeling hurt or affected.

I am truly sorry that you are still suffering more from something that happen some time ago and I understand that you can feel in pain due to all that yet, but I am going to send you here some ideas that can be helpful to deal with the past in terms of closing that door and focusing more in the present and the future:

1. The main thing to do is assess the situation and see if there's something else I can do about it. Can I change the situation? Should I say something to someone about it? Do I need to "pay a debt" to someone or something? Can I learn a lesson about it? If I feel I still should do something about it, I recommend doing it when it's possible and, obviously, if I really think it's going to be beneficial and not potentially problematic. Otherwise, I can only accept what it's like, even when it's a very hard experience.

2. In terms of acceptance, I recommend approaches based on Self-appreciation (Positive Psychology, gratitude exercises) and Acceptance (Mindfulness, Acceptance and Commitment Therapy, Forgiveness Therapy).

3. Explore and clarify your values and set your goals for the future aligned with them. Try to aim for the short-term goals that allow you to achieve the long-term ones and focus on the process (being present in the day-to-day stuff).

I honestly hope you can go over this situation. Having a plan and believing in yourself are capital things to achieve it.

Wish you all the best.