From what you describe, you've thrown yourself into your work but you don't feel really challenged or stretched. You also seem to have devoted yourself to your work (a plus--I get it) while ignoring some areas of your life that seem important (friends, hobbies).
Balance is really a key here. Work drives us. It provides rewards that can be quantified. But it isn't everything. Make time for self-care. Take a break. Accept that stepping off the treadmill is ok (essential, in fact).
Take time. Laugh. Goof off. Focus on relaxing. The earth (and the start-up) will continue to spin without you for a few hours.