If it's the long hours that are getting you down then turn work away, I'm busy all the time and at times it get too busy and I start to get stressed out, so I let all the calls go to the answer phone and screen them, I pick the best and call them back, so existing customers get called back and jobs that I want. I find the crap calls don't even leave a message.
When someone rings It's not a crime to say.... "I'm Sorry I'm not taking on new clients at the moment, I can reccomendation another company who can help you"
Might be an idea to take on a helper which I find really eases the day.