How to deal with work related stress?

Usual stress related to work overload is something I learned to tackle. I learned to say no, to shrug it off etc. Now I have a situation where a trusted colleague accused me for not having properly organized a project we had together. I am flabbergasted. Hadn’t seen that coming at all. It’s true, I had to rearrange some meetings because of other duties. I thought that everything happened with her agreement. Now I am invited for a talk with the boss. I guess they will tell me that we have to find ways to organize better etc. So, I am not due for firing. But I know that they keep a record and this will be remembered. I also feel bad because I always stepped in for my colleagues and now I feel my loyalty is betrayed.
Besides more practical issues - how to deal with the situation strategically - how do you manage emotionally? I just came out of a phase with on and off pain. Having learned a lot about myself and thought I reached a more calm place. And now I see my new calmness going down the drain. On the other hand, what is calmness worth if it doesn’t have an impact on a situation like this? And I am really angry, very angry!! And a bit anxious.