When you are hard on the inside, it is difficult to forgive yourself. Therefore, the key to forgiving yourself may be to change your inner atmosphere.
Start loving yourself by taking very good care of your body. Rest well. Eat well. Dress well. Drink plenty of water to keep your body hydrated. Keep your body clean. Keep your body fit with proper exercise.
The more you do this, the more you will have a sense of well-being, gentleness, and lightheartedness. Continue nurturing them as this is changing your inner atmosphere.
When your inner atmosphere is mostly gentle and lighthearted, try to forgive yourself. It’s usually in these inner atmospheres that you naturally forgive yourself and let go.