Letting go

I have just come off a project where a great number of things went wrong. Without going into details (I may cover later - or not, given the topic of this post) I found myself fuming most evenings.
It was apparent to me very early on that we were rushing head-long towards the edge of a cliff, but could not get any of the managers to agree. By the end of it, I had been completely marginalised and could say nothing without it being ignored or verified with someone else.
This is despite the fact that most of the concerns I raised blew up at some point and led to major issues. Indeed, throughout the project, we spent more time painting over the cracks without ever addressing the cause. The result is now in production and as beset with more problems than I expected - and still no-one is addressing the underlying issues, just patching the immediate failures.
But that is not the point, and yet again I find myself getting off topic.
The question is why I spend so much time angry and stewing about it. There are probably a number of things I could have done - although what is still not clear. But going over and over what should have been was never very helpful. Even more so now that I am not even involved any more! There is really no point in getting upset over an injustice done over a year ago when no-one else involved even remembers the situation.
[It is quite apparent that our managers never recall the past and only ever look a short distance into the future. This is not right but neither is it something that can be fixed by complaining.]
So, chalk it up as a learning experience (and I DID learn a lot) with one of the major learnings to be when to let go and when to push harder.
Now if I can only put that into practice...