Throwaway, this sounds like a classic case of burnout, and I don't know how to say this in a way that isn't going to sound rough, so here it is: You're doing your company a disservice by staying in your current state.
If you're unhappy with your work, you won't be passionate about leading a company. Especially through an acquisition, your colleagues are looking at you, a founder, as a leader, and drawing on you for strength. If you can find it within yourself to be that leader, then that's great! In that case, you probably shouldn't also be the lead developer, and given your extensive domain specific knowledge, it sounds like you might be doing too much all at once. I can't know; I'm not in your shoes, so this is all an outsider looking in.
It sounds like for the moment, what you really need is to pull back and relax, take some time for yourself, and recover. Your body only has so much willpower to go around, and if you get in the habit of exhausting that regularly, you'll burn out every time. Figure out if reducing your role at the company will let you continue, and do that if you want. Or, if you need to craft an exit plan, do that as well, and find someone just as passionate as you to fill your shoes. But take care of yourself first!