If its any comfort this isn't even unique to startups. Every software project I've ever done has felt like this. The list of possible tasks to do always far outpaces the resources on hand to do them.
I figured out the focus and priority part, but that angle, each person, is still an eyeopener.
It's something I've been struggling with, I know what the priorities for the product are, but not everyone can contribute (equally) to those priorities, and I don't want to make them feel like their work is less important.
I think looking at it from this angle will help a lot.