Wow, that's tough. I have heard that when a biz scales, sometimes the early employees don't fit as well, because the skills needed are different than when first starting out. To get things going you do a lot of everything, and later they need specialists. How passionate are you about this startup? What does your co-founder think? You would think there could be some role you could fit into, maybe COO? Depending on its success I would hold your ground , or they should be willing to buy you out or something. Best wishes!