The "sunk costs" situation is actually a little more complex than it might seem at first:
It looks like you have invested time and money in this opportunity and you ask yourself if it is likely to pay off. The intuition to stick to it even if there are hard times does actually have a sensible foundation: If the business took off in a couple of months without you being part of it, you would probably regret having left it.
So what you really want to evaluate is the likelihood of the business taking off within a timeframe that's tolerable for you. Startups have the property that not all progress is immediately rewarded. Some companies have to go through tough times until they hit gold. You now have to decide whether you think you have such a company or not.
As you obviously have more knowledge about your own situation/company than I do I can't assist you directly in that decision.
However, I remember reading a rule of thumb that if a startup does not have any significant traction (users or customers) 1.5 to 2 years after its foundation it is likely that there is something wrong with the business or product. I think this may have some truth in it.
It looks like you have invested time and money in this opportunity and you ask yourself if it is likely to pay off. The intuition to stick to it even if there are hard times does actually have a sensible foundation: If the business took off in a couple of months without you being part of it, you would probably regret having left it.
So what you really want to evaluate is the likelihood of the business taking off within a timeframe that's tolerable for you. Startups have the property that not all progress is immediately rewarded. Some companies have to go through tough times until they hit gold. You now have to decide whether you think you have such a company or not.
As you obviously have more knowledge about your own situation/company than I do I can't assist you directly in that decision. However, I remember reading a rule of thumb that if a startup does not have any significant traction (users or customers) 1.5 to 2 years after its foundation it is likely that there is something wrong with the business or product. I think this may have some truth in it.