That changes things a bit. If you believe strongly in the startup's strategic goals, then perhaps you can convince yourself that your current goal is purely tactical, and can indeed force yourself through it with willpower alone. Sometimes -- rarely, IMHO -- flogging yourself to get through some necessary shit work does make sense.
If you were actually working at the startup, on its principal mission, then your problem would be more complex. Not only would you have to think about the appropriateness/advisability of the particular feature you're implementing, but you'd have to constantly evaluate it in light of your relationship with your SO and your prospects in the new company. It would be very difficult to separate these questions in your mind, so IMHO it's a good thing you don't have to.
If you were actually working at the startup, on its principal mission, then your problem would be more complex. Not only would you have to think about the appropriateness/advisability of the particular feature you're implementing, but you'd have to constantly evaluate it in light of your relationship with your SO and your prospects in the new company. It would be very difficult to separate these questions in your mind, so IMHO it's a good thing you don't have to.