How do I let go of pent up anger towards an ex?

Don't dump on your ex. Your relationship is over. Write a letter to yourself, laying out all your grievances, then destroy it.

One technique that I've developed for dealing with anger towards exes is to write them a letter, and as soon as you finish it, destroy it. You can say anything you want this way, and you'll be able to organize your thoughts in a clear and logical manner.

In my opinion, you should divert your mind to something productive, something that would benefit you. In that way you will eventually forget about your ex, and that would be the best way to show them that their presence does not matter to you. It is the best way to show them that you never needed them. And, in my opinion, it's the best way to make them feel guilty or regret it.

If you're still communicating with that person, tell them that you need time to move on. This pent up energy will only cause you pain and sorrow. The last thing you want is to remain at that stage for too long. Give yourself time to heal and do things you love. Exercise is usually a great way to release that anger. Jogging, biking, even kickboxing. Don't have or keep those angry feelings, it will just make you feel worse about yourself in the future.

Try to remember the good times you guys had. It might hurt remembering but it'll help you to let go of your anger

I personally believe taking a break from speaking to this person may aid in letting go of some anger. Venting to a professional or a listener may also be beneficial.

Let your ex go. Breathe deeply each time you find yourself feeling the anger all over again. Just keep the good memories with you and be brave enough to start anew.

Try doing activities that tend to calm you down. Reading, writing, exercising, etc. can all be healthy ways to release anger or stress.

You've become much more and grown much more from where you were with them, Holding past anger is only bad for you and the day will come when you no longer feel it

It may help you to know that anger really only punishes you, not the person you're angry at. People make mistakes out of stress and/or ignorance, not malice. If you can look into what's making you angry about the other person and really see this fact, then compassion will come and anger will dissolve. Generally. Sometimes you're just going to stay angry :-(