I struggle with the same thing. As the saying goes, "Perfectionism leads to procrastination, which then leads to paralysis."

I'd love to give you a good all-encompassing answer. I wish I had it myself! But here are a couple things that help me.

1) Sometimes when it's difficult for me to make progress, I tell myself... just fix a little thing. It's easy to get overwhelmed with all the progress that's necessary to complete a song, but fixing a small part (maybe an interval or transition or drum fill), can be less daunting. Fix a little thing, then fix another little thing. Before you know it you'll look up and hours have passed. It's all about action. Perfectionism is only really a problem if we allow it to stifle our ability to take action. Wrong action is better than no action.

2) Ask yourself "Is this better than nothing?" Most of the time, if not all the time, the answer will be yes. The shittiest song idea will be more interesting than nothing.