Idioms that mean "to tell [someone] everything." or "to tell one's innermost feelings and secrets"..?

I have been looking for common idioms in English language and culture that convey the meaning of “expressing oneself, or telling [someone] about one’s innermost feelings and secrets.. etc.”.

I know the idiom “let [your] hair down” though, but I feel it’s a little feminine (since mostly females let their hair down, ya know..). So I think I could use your help with that, since all/most of you are native speakers of the language.

Thanks very much in advance.