So my take on it is that your only job as a startup if you don't already have product/market fit is to find it. And then, if you have it, you focus on growing.

Then, quite simply, your options are a) Find product/market or b) Quit.

To decide between a) or b), try to think how close you are to getting it. Do you have a few users that love your product but the pricing is wrong? Did you find something interesting in the market that you think you can compete on but didn't execute well-enough?

I would say if you lost your passion and don't think you've got something interesting enough to get a few users to love your product, then it's time to quit and (maybe) start over on a fresh idea. If on the other side you're still passionate about the space and think you're onto something, then keep trying.