I have found that I can hold laser focus for four hours. After that, I start to melt down. You might be similiar, if that's the case, then try to set your max hours at 4 hrs per day for client work and then set your rates accordingly.
If you find yourself working insane hours because you are getting a lot of demand, then raise your rates.
If you find yourself having to work insane hours because you need that many hours to pay your bills, then raise your rates.
That's the secret of being happy as a self employed developer. The less hours you work, the more productive you can be for those hours and the easier it is to stay disciplined. Otherwise you just become a overwhelmed, burnt out, procrastinating mess.
Imagine disappearing into a coding loop at around 8:00 AM and then emerging with a ton of stuff done at around noon. You have the entire rest of the day to do whatever you like and you were probably more productive than an employee chained to a desk for four hours.
An office is great for a change of scenery, but it's tough to justify the extra expense. Better to get out to co-working spaces or similar for that change of scenery. That means you are going to be working from home at least part of the time. Setup a desk in a quiet area which you only use for work. Don't use it for browsing, personal use, games, etc. Only for work. If you want to do personal stuff, then grab a tablet and step away from the work computer. When you are done for the day, then shut the computer off and forget about it until the next day.
As an entrepreneur, you should be spending some time working on business strategies. Take some time for some fun stuff which could also make you some money on the side. After your four hour grind, perhaps you can kick back a bit and work at a slower pace on alternative income streams. Make progress every day, but don't allow the side project to drain you (which will then affect your client work.) Rather, the side project should invigorate you. Hopefully it will also bring in enough extra money so that you aren't 100% reliant on client work and perhaps even allow you to further reduce the time you have to spend doing that work.
Not all of these are practical for all situations. Sometimes you just can't get away with only doing 4 hours per day of client work. Sometimes the client demands more time than that. Sometimes you have to work longer for financial needs. But I think these points are what you should strive for.
Also force yourself to work in a schedule. Get up at the same time and "go to the office" even if that is just your work area. Take a lunch break and then stop in the evening. If you don't force yourself into a schedule then you will quickly get overwhelmed with the mountain of work to do.
It takes discipline but can be done and when you get the balance right it is way better than working in someone else's office.
I will give you only one advice.
Think, you are still employed. Wake up early, take a shower, breakfast, change your clothes and start working on your favorite desk...
I'm like gexla and can only stay focussed when working at home for about 4 hours but in that time I'm very productive (more so than I am in an office)
If you're the same then you should work out what time of the day is your most productive. Mine is over lunch time, which includes the time when the baby is having a nap.
I recommend closing every tab in your browser which is unrelated to the task you're working on as open tabs can be distracting.
I also agree that you should set up a desk at home and only use it when you need to work. Turn the computer off when the work is done for the day.
Now, I do other stuff (laundry, errands, physical work) in the morning and start on the computer at noon. Works much better.
Sleep? For me, some weeks I'll live on 4 hours a night, but then I get super depressed and will jack up the sleep to 6 or more. I need to improve on this, so I'm probably not one to take advice from.