Some of your ennui might arise from being too close to the project. You've been working on it for months, you've been close to it, you've made compromises, and now you can't separate yourself enough past choices to be able to objectively evaluate your creation.

Once your project starts getting promoted, can you not iterate? Is the direction you think it needs to move in completely inimical from the version you're thinking of as 'finished'? Can you keep evolving it post-launch?

Do you like your partner? Is this someone you want to work with in the future? Have you discussed your objections to the direction? Have you been expressing your reservations, or will this come as a shock to everyone involved?

Is this something they'd launch even if you walked away from it?

Based on what you've described, I'd be inclined to let everyone know I have deep concerns, get it done, and start work on version two immediately. If at any point in that process I felt too large of a disconnect with the rest of the folks involved, I'd start extracting myself as politely and efficiently as possible.