I've been having this experience quite often now that I'm in Japan, trying to find work or converse with people. In interviews, I am absolutely terrified of messing up my speech...so much in fact that I go into vapor lock and can't think at all. Naturally, this looks incompetent to the person doing the interviewing. Everything in my mind either downgrades to baby babble or to English. When I'm angry or stressed, I can't help but switch to English. The exclamation of curse words in the mother tongue provides waaaay too much relief.
I have a French-Canadian friend who, though adept in Japanese and English, will immediately go to French and "breathe dragon fire"--as we call it.
Seriously, I've tried this not too long ago. I sounded like a dying cat and ended up doing more laughing than being angry. Needless to say, the issue was mitigated and I decided I should probably practice expressing anger in front of a mirror first hahah.