You're at your wits end, you need a break. Try not to make a serious life decision when you're overworked and stressed to the max. Clarity first - go for a long drive, swim, run or workout. Do something to clear your mind and push out some stress. Physical activity highly advised.

I've worked in a similar environment and even though there has been a lot of research of the dires and productivity loss of exceeding the 40hr work week, we still push ourselves. If you feel this way so does some members of your team probably. The team loses if you all don't take a breather or lighten your load. I don't know the best route - hiring more employees, take less customers, under promise and over deliver on timetables, charge more, etc but a conversation with you partner may help.

Google some of the research done on exceeding 40hr wrk weeks and maybe you can find some better direction.

You can get through this. Best of luck.