You can't change the past, but you might be able to reframe some things.
For example, all the times that were painful, you were learning first hand compassion.
All the times you were naive, you were gaining wisdom.
All the times you were mean or cold, you were learning regret and the value of kindness.
All the times you believed someone who was deceiving you, you learned to trust your intuition that was trying to tell you something was wrong all along.
All the times you were too afraid to do or say what you really wanted to, you were learning the value of honesty and straightforwardness.
If you look to the past at all the pain and mistakes, you will feel small and ashamed.
If you forgive yourself and others in the present, you can live in the present and look to the future with confidence and wisdom to live life better than you ever have before.