Here are three things you can do to let go of an ex. partner (or help your partner, if they have this issue):
1. Develop your confidence and self-esteem so that you can break your dependence on your partner. You have the very gifts that you looked to your partner to give to you.
2. Recognise any resentment or hurt you are still feeling towards your ex. and work to forgive them for their behaviour. Reach a point where you can genuinely bless them in their new life.
3. Identify any guilt that you might be hanging on to because of anything you did/did not do in the relationship that lead to its failure. Forgive yourself for this - you did your best and can now learn from that experience.