25-year-old model Karrueche Tran promises to stick by Chris Brown "no matter what"-- she will be there for him, even if he is sentenced to more jail time.

A source said, "She wants to assure him that there's nothing to worry about and that she will be there for him no matter what, even if worse comes to worse.”

Chris is currently in jail because his arrest for the alleged assault last October, but he confided in Karrueche about how tough he is finding life in prison.

The source added, "Kae couldn't believe it because he's rarely ever emotional with her. He told her he's tired and that he couldn't sleep and how he doesn't know if he'll be able to fall asleep because he's nervous.”

If convicted on the assault charge, Brown faces up to six months in jail and $1,000 fine, as well as up to four more years in jail in California for violating his probation.