I have been in a similar situation. First step in my opinion is to go on a holiday, at least 1 week long (ideally 2 weeks or slightly longer). And get away from a computer. Head somewhere where you can relax. Working on a side project is not a holiday, its still working. Get outdoors, exercise, use tips you can find online for beating depression like characteristics.

This is to give you some perspective and time to decompress from the day to day pressures at work & regular home life.

Your burnout is going to be clouding your view and judgement of the situation at work. The time off will help you look at the situation with some fresh eyes hopefully.

If the idea is worth millions, what sort of stock options do you have? How vested you are is a consideration. Also can you afford to exercise those options? Would you be financially better offer continuing to work and vesting options to exercise later (assuming you're in the USA under their style of share plans). If you don't have stock options (or something equivalent) well then jumping ship to another job is probably the best course of action (maybe).

What are your motivations for working in this job? Is there the potential of a large pay day if the company has successful liquidity event under favorable terms? Get to build X and have your name on it? Understanding those will help you work out what you want to do and why you want to do it.

You need to talk to your boss as well. Why is the feedback bad? What are actionable things you can do to improve the feedback? Are you not working well with others now that the team is growing?

Talking to the founders about some struggles you're having may help, but how good is your relationship with them? How understanding of the work you have done are they? If they're not across the detail of what you have delivered this may backfire depending on their personality types etc.