What happened when you tried working a 40 hour week as an experiment? Did your business crumble and force you to work 120 hours the next week just to put it back together? Or did things work out fine? Or have you not tried that?
If you haven't, what do you expect would happen if you did?
From experience, I can tell you that things will work out fine. You'll get your product shipped, support your customers, and have roughly the same chance of surviving if you work sane hours.
When I was crunching to ship Twiddla, I would average between 20 and 30 hours per week. About the same amount of time I dedicated to Rock Climbing, wandering the streets of Pamplona, and sipping wine on the balcony watching the world go by with my girlfriend. Every work day I would come in fresh, with a good idea of what needed building, go heads down in the morning and come up for air around 3pm to find a cold half cup of morning coffee on my desk, next to a cold lunch that my girlfriend had evidently snuck in and deposited at some point.
That was the most productive year of my life. I shipped a product that pays for my lifestyle today. And I never worked more than 40 hours in a single week.
I'd recommend you give that a shot. It sounds a lot more enjoyable than what you're doing.