Theory of Games
Storied Gamist

This is shocker for the Gamemasters out there, so players can take 15.
What do you do when you're running an adventure or you're early into a campaign and realize you are NOT feeling the game system? Some rule or collection of rules is triggering your "NO FUN" sensor. Or maybe the setting feels wrong - you're in the Middle Ages but your really wanted something more Iron Age. Worse still, the players all seem to be enjoying themselves. It's a tough spot to be in and I've been there a few times - it can be a real crisis at the table.
I tend to hit the brakes and let the group know I'm not enjoying things, and I explain the problem to them. Then we usually make whatever adjustment works and move on - "move on" meaning a campaign restart with slightly revised expectations. Usually if there's a rules issue, I toss in a house-rule that works for the group (me mostly). There's also been a few times where we don't move on and the campaign ends completely, sometimes abruptly (depends on the group). I guess I never feel the need to continue running something I don't enjoy, which can suck for the players unfortunately. But, I've heard of GMs hanging in there despite their issues, so I wanted to get your experiences and opinions. I'm always trying to be the best GM I can and that requires the ability to learn from others.
How do you handle rules or setting discomfort at the table? Am I wrong? Any horror stories? Best practices?