This has often been a problem for me, too.

Right now I've settled on a few techniques that help me out. One is to use the Pomodoro Technique, which is working in 25-minute, focused bursts with 5-minute breaks in between after completing a task. I also use a simple to-do list app where I can jot things down with a due date.

I try not to overload myself with things that aren't important and can be skipped, too.