By pursuing this startup, what is the value of other opportunities you have to pass up. If you think the value of the startup is higher than those, then stick with the startup. However it sounds like you don't feel that way. If the other opportunities are a higher value, you should pursue those instead.
So far it sounds like you are mostly out time as you can get your cash back. If the business is not profitable and you don't have a good outlook on it. Then your time value so far is is lost, or sunk. Sunk costs, are sunk. You can't get them back, and people have lost a lot more trying to get them back.
As others have mentioned about, try to negotiate are prorated amount of your shares. If you can't get thank, don't keep wasting time here. Cut your losses and learn a lesson from it, then move on.