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

You accept that what happened is all in the past and that you cant change that but you can change the future:)

Think of him/her as someone who doesn't hold any value and it would be worthless to waste your time and energy on him/her by steeping down at his/her level. Otherwise, you could just take a pillow and hit it hard until it's shattered. Think of the pillow as your ex.

Do something you love,you have to take your mind out of it.Go out,shopp 'till you drop,marathoon watch a movie,read books,try to make anything else a priority!

Do the things you wanted to do in the first place. Only you get to define who you are. Forgiving them is solely for your benefit.

I wrote my list of grievances in my journal, over and over, as well as letters to him (that I wouldn't dream of sending) and to a third party in our relationship. That got a lot of uncomfortable stuff out of the way and showed me my own part in it all. We can go on being their victims or we can understand that we contributed to the situation simply be being there. Someone wiser than I said: "We teach other people how to treat us." This is so true in an abusive relationship, so we have to acknowledge what we got out of it and why we stayed with it for so long. I realised I was very angry at myself for letting him. Had my self-esteem been all it should be I would have run a mile very soon after meeting him. So then you go through "What an idiot I must be" and all the self-doubt that follows. Don't get stuck there; it's a waste of time.

Although your ex left you, your ex doing that made you stronger and left you an opportunity to find someone else who can make you even happier then your ex ever could. For this there is no need for anger if anything be grateful.

You will learn to let go when the time comes, Time doesn't heal your pain nor cool your anger, it just numb your feeling and you feel empty in them

That just takes time.. If we don't dwell on it, it starts to dissipate.. Disconnect yourself from it all..

Acceptance is a MUST. Just accept the fact that whatever happened is done and anger doesn't help it.

The relationship ended for a reason (or reasons). And no one person is 100% responsible for it. Each person contributes their share. So its important to learn from the mistakes one made, learn from the lesson that you got out of that relationship and use them as a stepping stone to move on to a better one. Looking back doesn't solve anything. Trying to channelize the rage into something positive like volunteering, helping others etc will help in reducing the rage.