From what I've read, I feel like you have the ability to weld considerable power to change your circumstances. You are the most senior engineer at a technical startup. If you leave it could kill the company. At the same time the management might not have the wisdom to see that.

What is making you feel burnt out? Long hours? Management? Long commutes?

How can you mitigate the feeling of burnout in the short term to give you some room to think and plan your next move? Could you take a holiday. Could you work less hours? Could you bring in healthier boundaries with your manager and colleagues.

If you can alleviate the stress in the short-term, it could make it easier to find another job/opportunity.

If you see no way of doing this, then quit. Your health is your most important asset.

Believe in yourself that you can find something and the future is bright. :) You make your own luck and fortune favors the brave.