When I have a hard time deciding, I go grab one of my "in progress" totes at random and start working on it. If I stay engaged, it might get done, and if not it gets put back in the tote and back on the shelf. Sometimes by then, an idea has come to the top and I'm motivated to start on that, and if not I grab something else.
If I spend too much time looking at the different projects, I can do that for days before I start sewing, so I don't do that. I don't let it bother me that I've got a lot in progress, because that means I've got a lot to keep me busy if the time comes when I can't afford to buy the new stuff.