Here's a very tentative hypothesis: I get angry when things don't work right, when systems break down. But when people let me down, that's already in the equation. I expect to be forgotten or pushed to the side some of the time. It's not like there's any friend who never comes through for me; everyone does, in their measure. I've learned, however, that those measures are limited—that if I want to really be able to count on something, I have to make it work myself. When I can't do that, that's when I get angry.
In sum: I expect relying on others to eventually be futile, but I expect that I can make things work. That's pretty screwed up in multiple directions.
So today I was tired and gave up after a phone call. I'd rather let it go than to continue to bang my head against the wall and ruin my chance at a little rest. There'll be time to be angry later, I suppose, if I need to be. I'm not denying that there's disappointment and hurt; I just don't want to make it bigger than it is.
I'm more bothered and hurt now that I've written. That wasn't such a good idea, perhaps.