Variants include LTSFI, which can stand for Let This Shit Fuck Itself, OR Let That Shit Fuck Itself. A more aggressive variant is LTSFM - Let That Shit Fuck Me. This slightly changes the connotation from things just going to hell, to things becoming an active problem in one's own life, screwing with one.
OtherDude: "Man, relax, it's not our problem. LSFI."
Guy1: "Dude, your bank account if overdrawn, your credit card is near its limit, and you have barely 10 dollars in your wallet... hadn't you better get your shit together??"
Guy2: "Meh, forget it man. LTSFM."