I know exactly how you feel. Having to show up to an office after working at your own startup is challenging, writing pointless unit tests and doing what a boss tells you after years of cowboy coding sucks. Take a short break from work (2-3 days) and catch up on sleep and any errands that make you stressed. Evaluate your life after that. Startups are a huge grind sometimes, and it's totally normal to burn out and hate it. If you're a cofounder, you should expect to burn out, otherwise you weren't working hard enough. After you take a break:
- you're valuable to your startup and difficult to replace
- the more traction your startup gets, the better your next job
- cofounder disagreement is normal, otherwise you're not engaged enough
- address any issues and don't be afraid to ask for justification if you disagree with anything, after all the value of a startup is to teach you how to make hard decisions
- is there anything interesting you think the startup should be doing, but isn't? Do it!
- is there anything marketable that you want to learn, but it only relates to the startup tangentially? Do it!