Real progress in understanding nature is rarely incremental. All important advances are sudden intuitions, new principles, new ways of seeing.
The reason I stop playing songs is usually because I get sick of them, and then they find themselves back into the set list at some point.
Don't be afraid to give your best to what seemingly are small jobs. Every time you conquer one it makes you that much stronger. If you do the little jobs well, the big ones will tend to take care of themselves.

