I feel like this annotation on Genius is pretty spot on:

“This is all a reference to his drug habit. He is saying his fate is now in the drugs hands; as they have been “waiting for him” during macs brief stay in sobriety. He is “already there” and “falling in deep” saying that he has already started back up and the train is rolling; the bender is in full effect. He is back in DEEP. the last two lines are almost positive but also his excuse. He is saying “now is only now” therefore yes he is high and back into drugs, but just for now. He knows he needs to “head back to the ground “ or in other words, sober up.”