To be honest, I’d say learning to let go of the fact you can’t change what made you bitter, though this is the hardest step. Forgiving yourself, while not necessarily forgiving the other person. Bitterness is fine to feel, but in order to let it go, you need to not let it devour you. Feel it and let it flow back out of you, take that energy and then give it back. You are worth not having to feel bitter or miserable towards them and you deserve better then how they made you feel.