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.

I think mnw21cam's point still stands - if working at your current position is "seriously affects the quality of [your] life", you need to prioritize that over making money for your boyfriend's startup, or you'll hold it against both him and the company for making you do something you don't want to.

Take care of yourself first, even if it means a slower launch for the startup! And it sounds like you have good coding practices and are well-regarded at your current position, so hopefully you could find another job that is more aligned with your values. Best of luck!