Never work for free.

The entire purpose of working is to exchange your contribution for something of value. Whether that value is an intellectual reward, money, equity, etc. is up to you. It seems that in the beginning you valued the intellectual reward the project gave you, but that is now dwindling.

I would have a frank conversation about your compensation and IP, and be prepared to move on. Don't let your emotions get the best of you, this is business.