forget oneself
verb1
Idiomatic to lose one's dignity, temper, or self-control.
- "Look'ee here, Pip," said he, laying his hand on my arm in a suddenly altered and subdued manner; "first of all, look'ee here. I forgot myself half a minute ago. What I said was low; that's what it was; low. […]1861, Charles Dickens, Great Expectations:
2
To become unmindful of one's own personality; to be lost in thought.