To tell the truth introduces an admission you might have hidden. To tell the truth, I had forgotten.
It signals frankness rather than proving it, and what follows is usually small. The fuller form, to tell you the truth, is just as common.
1phraseto tell the truthIntroducing an admission you might have hidden.
“To tell the truth, I never liked the place.”
to tell the truth is a phrase.
Practised in Small Talk & Everyday Phrases