The jump from 3+2 to 3+0 hurts because you've built a skill set optimized for one specific constraint. With increment, you get breathing room to calculate variations and spot tactics you might otherwise miss. Without it, you have to play on pure instinct and pattern recognition - which you haven't trained as hard since the increment was doing half the work. That's not wasted time though; those months taught you calculation discipline. The issue is you need both skill sets, not just one.
Here's what most people don't mention: don't force yourself to 3+0 cold turkey. Your brain needs a transition. Drop down to games like 2+1 or 1+1 where the increment is still there but the base time forces faster decisions. Spend a couple weeks there, then try 2+0, then 3+0. You'll adapt way faster than jumping straight down because you're gradually rewiring how quickly you need to evaluate positions. It's like going from blitz to bullet - you wouldn't jump there either.
The other thing: when you do play 3+0, expect to drop maybe 100-200 rating points while you adjust. That's normal and temporary. You're not losing skill; you're just exposing a gap in your speed. Once you get comfortable with the pace, that rating will climb back faster than you think because the calculation habits from those 3+2 games are still there - you're just accessing them quicker now.