If you’re having trouble fleshing out your characters, continually ask yourself in each scene, “What does this character want?” (Yes, it’s as simple as that!)
Say this out loud enough, and soon someone nearby will ask why you keep repeating that. Do not reply, but simply keep questioning aloud, “What does this character want?”
Eventually you’ll be committed to an asylum. Asylums are great places to think without the distractions of the modern world. I’m sure you’ll figure out that pesky protagonist in no time.
Trust yourself that the motivations you want for your characters is correct and will help complete your vision of the story.