I have worked at the same startup for 4 years now. During a meeting last week I realized: I don't care about our product or it's mission anymore. I would rather be watching TV with my girlfriend or literally doing anything else.
I think I lost my faith. It's not burnout, I work 40hr weeks. But lately I just don't give a shit about bugs or features we're building.
What do I do? How do you deal with this?
That's really ok.
Now you've figured out what you really do care about (or at least a piece of it) - spending time with the people you love. In many ways that's a far healthier perspective to have on life.
So keep going to work and doing your job because, hey, you've got to pay the rent. If the only reason you're at this particular job is the company's mission, then start looking around for a place that pays more or has a better culture, or where you can learn more interesting things.
And in the meantime, keep doing a good job simply because you take pride in your work, and most hiring managers outside the weird "have to be passionate" startup scene will be glad to hire you.