Work-life balance is really important, and having a safety net outside of your workplace and project is key to not going completely batshit.

What's really bad is to find yourself in a position where you see your cofounder/coworker the vast majority of the time, and your work/project gets tangled up in the normal ebb and flow of the relationship.

You really, really don't want to get into a loop where you need to decompress with your best friends, but you can't because you avoid them because your shared work is going poorly, and the work is going poorly because you can't decompress. Yeah, being in that position sucks.

Make sure you've got something positive in your life beyond your current project.