Whew, took me long enough.
In this game, if I get stuck on a level, I can switch to one of the other six open worlds (after beating world 1) and continue on it, or start if I hadn't entered it before. So if I'm stuck, I can try a different level.
For a time. I was stuck on a level in worlds 6 and 7, and had cleared every other world (except 8, which I couldn't reach until 6 and 7 were both out with 100%).
I'd been fighting those levels for a while, but I finally beat them!
The game is "Mole Mania." It is the third game I ever bought for my Game Boy- the black&white edition.
I had been stuck on those levels for three years.
w00t. I'm now in Boss Refight Mode, World 8, trying to finish the game...
..geez, they made round 2 a lot harder, didn't they? Ouch...