Learn English – What does “I want to sleep on it” mean

It expresses two things:

• I strongly agree with you
• I (strongly) think you are right to tell him this

In other words, it expresses both agreement and support. Agreeing with the sentiment, and supporting the speaker in expressing that sentiment.

Probably you refer to a song by Eminem. Text here:

http://de.urbandictionary.com/define.php?term=lose+yourself

In the first line we have "You better lose yourself in the music..."

Here to lose has its original sense as in "Damn, I've lost my keys." OALD has "lose yourself in something" as an idiom meaning Forget everything else and become one with the thing you are interested in.

As a maxim of life of dubious value.

http://www.oxfordlearnersdictionaries.com/definition/english/lose?q=lose

You'll find "to lose oneself in sth" at the bottom of the page (phrasal verbs).