I really don't know where to begin, forgiveness is such a big subject. If you believe that God forgives you, you are well on the way and if you are also asking Him to teach you how to forgive yourself also will help.
I can only share my experience. I had to identify all the road blocks that stood in the way of forgiving myself. I had to start by removing all the things that stood in the way of accepting myself - the good, the bad, and the ugly. One huge roadblock for me was continuing to “play God” myself. I was my own judge, jury and executioner. I was taking on God's role and punishing myself for all that I had done wrong. I had to stop “playing God” and learn to become His child. I had to ask myself if I would forgive others in the program for doing the things I had done. The answer was always - yes! Of course, I did not judge them harshly like I did myself. I could see that their behavior and choices were made in their active addiction. I could see that they were not perfect. I had to stop expecting perfection of myself. That always led to me feeling like a failure.
Once I learned to accept myself, I had to begin to change the things I could change. Steps 4-7 are about learning to change what I could about myself. Finally, I had to put myself on the list of people I had harmed and had to make amends to myself. This is where the forgiveness really started to take hold. Yet, if I am honest, what has really allowed me to forgive myself is living a clean life. It takes time, but you are worth it. There are a number of good books on the subject. I would be happy to suggest some if you are interested. Also you may want to read in the AA Big Book pp. - 70, 86 and the Twelve and Twelve pp. - 57, 58, 65, 78.
May you find His peace,