As a person who had to learn to repress her anger, I understand the level of anxiety that can create.
We'll take this step by step, because it's a long, hard journey.
First - learn to acknowledge your feelings. Recognize when you're angry, and validate your anger. Literally, "I am angry about this, and I have a right to feel anger." You do have the right to feel anger. You may not have the right to spray that anger all over people above you in your life, or random people who have done nothing to deserve it, but you do have the right to BE ANGRY.
Second - once you start validating your anger, a lot of rage is going to start bubbling to the surface. Get involved in some type of controlled combat sport. Learn to kickbox, find a martial art studio. Anything where the movements are ritualistic, and energetic, and yet require control, will help. You want to get the energy out in a constructive way while also learning how to control your feelings in a healthy way instead of repressing them. Repression is not control, and control is not repression. Working a heavy bag helped me a little through this stage, but for awhile you're just going to be angry at everything and everyone, and you're going to have a pretty short fuse, because once you start expressing your anger, you have a lot of it to express, and because you've never learned a constructive or healthy way to deal with that particular feeling, it's not going to come out in constructive and healthy ways unless you take the time to teach yourself those ways. You may want to prepare the people in your life for the explosion. You also may want to prepare yourself for the sudden desire to burn your life to the ground.
Third - you have acknowledged your anger. You have learned to express it physically and under controlled circumstances. It's time to learn to verbalize it. This starts with writing, because that way you don't shit all over the people in your life who are important to you, or with whom you cannot afford to vent. Start simply. "I am angry." Go from there. "I am angry because..." Describe the full situation. Try to connect it to your past. The thing about anger is that it's a masking emotion. It comes out when you feel other things but don't feel that you have enough control over a situation to express those other things.
A few days ago the kitchen started to smell bad. I searched everywhere, and couldn't find the source of the smell. Yesterday while I was cleaning the microwave, one of my companions told me, "That smell is rotting potatoes." Well, I know where we keep the potatoes, so I immediately went to that cupboard, and sure enough, that was the source of the stink. Rotting potatoes are dangerous. The chemical they put off when they start to rot is deadly. Those potatoes could have killed our cats, even if the gas was dispersed enough to not harm us humans. I was angry, but also a bit suddenly shocked and afraid because WHAT IF! So I called my husband, and told him what the issue was (We have dead potatoes.) and how I wanted it solved. I set two boundaries. He needed to take out the trash as soon as he got home (the bags are too big and heavy for me - he wanted a larger trash can and we compromised - he could have his larger trashcan, but he would be the one to always take out the trash). Right now it's late autumn, early winter. The sun sets around 4:30pm here. Bird gets home around 7:30pm - so he would have to take out the trash in the dark, which he doesn't like to do, but he agreed that the potatoes had to go. My second boundary was that if we buy potatoes again, they have to stay on the counter where we can see them, so that they get used, or when they start to go, we can throw them away BEFORE they start producing the gas. I know that potatoes not stored in the dark go bad faster, but if it's a choice between storing them properly and forgetting we ever BOUGHT potatoes until suddenly they're putting our furbabies at risk, or letting them go bad faster but not having it be a health hazard, I'm gonna pick option 2.
So - I got mad. I cleaned up the mess, and that got rid of the energy of my mad. That let me think clearly again, at which point I went to the person who caused the problem in the first place, told him exactly what the problem was without blaming him, and without trying to make my emotional reaction his responsibility. I set boundaries, and requested actions, and suggested a long term solution.
This whole thing is a learning process. You're not going to be perfect right out of the gate. It's going to be messy, so you'll want to prepare people for that. Ultimately, you want to get to a point where you acknowledge your anger and the reasons for it, accept that you have legitimate reasons - validate your feelings, figure out if there's a root issue that's being ignored and if there's a way to solve it, get rid of the excess energy if you need to by doing something active and constructive, and then sit down and describe your anger.
I feel ----- because ----- and I need ---- to happen to solve this.
Work really hard to not make other people responsible for your feelings. Do not use words like, "you make me." It puts people on the defensive and it's not actually true. You react to situations - no person is making you react, your history makes you react, your chemistry makes you react - you are reacting to stimuli - not unlike a tree reacting to the sun by producing oxygen.
I hope this helps.
-Raven