There's a lot in both the OP and in the subsequent comments that I can relate to here. But I've put off commenting for a few days as for me these are a lot of different issues that are being conflated here into a single monolithic thing called anger. It's a little more complicated than that. To try and simplify things, I'll break it down into four different themes.
1) Fleeting day to day annoyances - Here we're talking about things like poor customer services and those arsehole bus drivers that won't open the doors for you. These things happen all the time, will always happen, and is something that all types have to deal with. This is perhaps an issue for our type because the stereotypical NFP is perceived as a little bit of a pushover, and yes, it can fester inside you when you repeatedly get the worse of such situations. Over time I've become extremely assertive in my day to day dealings with world, although that doesn't necessarily mean turning into the Incredible Hulk every time someone spills my pint. You just deal with things as they arise, and more often than not I feel the best approach is to be calm and polite, yet implacable. Occasionally it does mean meeting aggression with aggression. If you deal with these things confidently as they arise, even when something goes against you it won't fester because you know you can't win them all.
2) "Theatrical" anger - Perhaps the part of me that other people least understand. Sometimes I just don't care about the big things, but small things really piss me off because there is an important principle at stake. The times when this can really grate is when I bite my tongue and keep quiet for the sake of the larger point, which might mean less to me but is more practical. For example, I recently got Apple to repair something on my laptop. They tried to charge me for it but I calmly insisted that I shouldn't have to pay, and I didn't. But a passing comment by the sales assistant really riled me and I wanted to lose my temper. But I bit my tongue and didn't say anything because I was getting my way and didn't want to rock the boat, which was the larger thing at stake. What annoyed me was Apple pretending that they were being magnanimous and great in doing this thing for me for free, whereas in fact I was merely asserting my consumer rights, and they would have gladly fleeced me if I had let them.
Okay, so that example wasn't a great example of "theatrical" anger, but the point I wandered off from was that sometimes when I'm getting angry about little things people don't take me seriously. Precisely because those things are so little, they think I'm being theatrical. One of the ENFP profiles mentions us as constantly going off against some form of bureaucracy and I related to it a lot because I used to work for a very bureaucratic company. On one occasion I lost my swipe card needed to get into the building, and to get a new one I had to pay a small amount. I kicked up a big fuss about this, and for a long time refused. My INTJ friend and co-worker kept looking at me as if I was insane because the actual cost of the new card was very small, but the principle that I had to pay in order to be able go to work - to make money for the company I was paying - was abhorrent beyond belief for me.
Conversely, sometimes I can be very deliberately theatrical when it comes to arguing about the "big political issues". Particularly as I am of Greek provenance, and calm debate among Greeks can come across as very impassioned to others. More phlegmatic north European types can interpret this as I sign that I'm getting really wound up and angry, when in fact it's all just done for effect.
3) Anger at things said and done in relationships (not necessarily romantic) - This is where I can be really avoidant and where problems can arise. Here I'm just not good at dealing with things as they happen. I need to mull things over, but by the time I have reached my conclusions it feels like raking over old muck to bring those things up, and I have no appetite for doing that. So things fester. And I can become quite passive aggressive, which is one of my least appealing traits.
4) The indescribable rage deep in your soul - I didn't even know this existed until a mushroom trip about five years ago. Then I just let go of it and everything made sense. It's just not there any more - it helped that soon after the mushroom trip I want on a couple of Buddhist meditation retreats where I consolidated a lot of what I had just learnt.
The one thing I would say here: be careful what you wish for. This anger really drove me on, and in some ways I miss it. It gave me direction, and much of the past five years have involved trying to find a new wind for my sails. I'm a happier person now, but I wouldn't say life has become easier.