@Felthry I don't know anything that works all the time, but I do know a few things that sometimes help.
The best I know is to have someone else spot what's happening and pull you back. Hard to set up but worth it.
You can dissociate, imagine that this is something that happened to someone else that you just heard about.
You can deliberately try to focus on a recent triumph instead.
The most important thing to know is that it's a pattern of distorted thinking, with all the associated implications.
re: mh - (not now), asking for help
@Felthry Even if you could, that hypothetical person couldn't always be there. It's powerful but unreliable.