come clean

сome clean (with sb) (about sth) (to finally own uр to sth bad that you have been trying to keep a secret) — выложить начистоту; признаться наконец

Example 1: She finally came clean (with him) about her affair. — Она наконец призналась ему в своем романе.

Example 2: I thought/felt it was time to come clean (with everybody) about what I'd been doing. — Я понимал, что пришло время рассказать начистоту о том, чем я занимался.

Example 3:
When Jeff approaches Marni’s desk to tell her he has started a college fund for her baby, Marni has finally had enough. She announces to the office that since they have been asking about her personal life lately, she would like to come clean. The father of her baby is John, her Internet boyfriend from years ago:
"This is getting crazy. It’s time to come clean. So, you’ve been a little involved in my personal life lately, and especially one of you. So I’m just…I’m going to tell you who the father is, and, it’s John." (englishbaby.com)

[open up]

see also
[get sth off one's chest]
[make a clear breast of it]
[out of one's system]