Mike 'The Situation' Sorrentino was addicted to pills for a long time during his heyday -- a dark period he's now shining a light on with his new book.

The "Jersey Shore" star's memoir "Reality Check: Making the Best of the Situation -- How I Overcame Addiction, Loss, and Prison" drops Tuesday, and dives into just about all of Mike's ups and downs during his TV career -- including his battle with drugs.

As it turns out, The Situation was actually hooked on painkillers for years ... much of which overlapped with his time on the hit MTV reality series, up to and including Season 5.

Mike came on 'TMZ Live' Monday to break down the infamous cement self-KO headbutt he did on camera, saying it was actually tied to him going through withdrawals at the time. Seeing how deep into the belly of the beast he was, MS counts himself lucky to be alive.

He also tells us the MTV team came to him often during filming and asked if he needed help -- but, at one point, his addiction got so bad ... he says they gave him an ultimatum, which is detailed in his new book.

We asked Mike what the lowest point for him was during this saga -- and while you might think his prison stint would be in the ballpark of contenders ... he actually says another time he tried a new drug was when he hit rock bottom. As MS explains ... he once tried heroin.

The guy has lived a lot of life, and has been through a lot as well ... thankfully, he's 8 years sober and ready to share his story honestly with fans. Lots to unpack here, no doubt.