How to forget the past is not easy for a lot of people but first you have to accept what you have been through. Once we accept, it causes us to not be bitter about what occurred over time. Also, having something to look forward to and having a mindset that new opportunities will arrive in the near future.
Not in much detail as I desire to, but from my understanding of what I learned there is a term called "schema". Schema is what our brain first processes when we are younger and absorbs from the first time someone speaks to us. Usually what we tend to grasp and be brought into remembrance about (usually negative statements or words).
How to forget the past is not easy for a lot of people but first you have to accept what you have been through. Once we accept, it causes us to not be bitter about what occurred over time. Also, having something to look forward to and having a mindset that new opportunities will arrive in the near future.