Britney Spears‘ ex-husband Kevin Federline is raising concerns about her in his new memoir … he says she’s not OK since her conservatorship ended, and writes, “The truth is, this situation with Britney feels like it’s racing toward something irreversible.”

K-Fed’s got a new memoir, “You Thought You Knew,” dropping next Tuesday, and he sat down for an interview with The New York Times and gave the newspaper an advance copy … in the book he writes, “Something bad is going to happen if things don’t change, and my biggest fear is that our sons will be left holding the pieces.”

Britney’s ex says it’s “become impossible to pretend everything’s OK” with Britney and adds … “From where I sit, the clock is ticking, and we’re getting close to the 11th hour.”

Kevin’s memoir recounts his marriage to Britney, plus their nasty divorce and custody battle over their two sons — Sean Preston and Jayden James — and Kevin says when the boys were younger, they once told him they didn’t want to go back to Britney’s house because they were scared.

In one chapter, Kevin writes … “They would awaken sometimes at night to find her standing silently in the doorway, watching them sleep — ‘Oh, you’re awake?’ — with a knife in her hand. Then she’d turn around and pad off without explanation.”