I think the problem is you went from 60 to zero and he's confused. Obviously, what you are now doing isn't working. All dogs are different. Some will work for praise and be happy with it. I was taught with my first dog praise only. And it didn't work.
So I look at it this way. If you went to work Monday thru Friday and on Friday they paid you. Then suddenly on Friday they decided..meh...work for praise. Are you going to come back on Monday?
There has to be a reward. It has to be a reward that works for the dog. I started training my boy with food. I used his meals. Then we moved on to a ball. We build duration on all exercises. But he always gets a reward at the end. He always wins.
As far as him refusing and you leaving the room? I would not do that. First rule always...don't beat it to death. If you get a sit at 30 seconds and that's what you wanted...you are done. Move on to something else. If he's just refusing to sit, then you make him. At some point, he has to learn that he HAS to do this. But if you correct him and he does it, reward him! Reward that sit! Reward the position! Often. Don't always put him in a sit and leave him. Sit! Reward! Sit! Stay! Reward! Sit! Reward! If it's boring and stressful he isn't going to want to do it.