What about letting someone take over the project but you get to keep a share of the profits for guidance and advice (30% or so).
It benefits both parties, you know what features the project needs to implement and what direction it needs to take to make more money, while the new owner/developer gets ramped up. You guys can even agree to a one year period of help and then separate, but at least the project keeps running and making money.
Not sure if this is usually done, but it seems like everyone wins :)