Consider to take a break for 2 weeks, rather than resigning, replacing yourself or talking to your wife. After 2 weeks you may feel less burnt out and see things more clearly. You can get medication (and I for one can confirm it does work) but ideally that's to get you out of the rut not a permanent solution. It's possible that your lack of passion for this idea, combined with how much time and dedication it takes is making you feel this way. Your mind/body doesn't understand the inherent contradiction. I'm married to my cofounder and it's hard to be honest when one is feeling weak, but hiding stuff damages your intimacy and closeness. Protect that, it's worth more than your company.