My guess from here is that it's not some sort of deep crisis, just a buildup of pain points that's gotten up to your neck. If you can get a few things sorted out, the picture might turn nice and sunny, and your "but I just want to RELAX!" ennui will vaporize. But if you don't sort them out, you'll burn out, let everyone down, and be miserable. Remember: having a lot of clients is supposed to be a problem you want, but you need to make sure you're equipped to handle it.
Here's my advice from having burned out terribly and repaired it:
Take tomorrow off. Send the email out right now: "Guys, I need a personal day. I'm sure you can handle it." Sleep in. Watch some TV for like half the day. Whatever.
The problem is likely that you've been hill-climbing. What's the next problem and how do I solve it? The hill gets steeper as you go up, but you haven't had the time to find a better path up, or maybe even a different hill. What you'll need to do is take a step back and ask, what's the problem with how we're solving problems, and to do that, you need to hit the pause button on everything. That's the real reason you just took tomorrow off. So after you're done with TV...
Part of your job is to create sustainability. Working like crazy just isn't sustainable and if you burn out, you're not likely to be the only one. Fuck toughness; it's not even a thing. And fuck the superheroics, because people just don't scale. You're not trying to lift a huge mountain yourself or even with a couple of your buddies; you're trying to build an organization that will systematically take apart the mountain and ship it off in modular containers. How do you turn "I need to make 1000 decisions today!" into that?
1. Delegation. Make a list: why does your phone blow up all day? "People need me to make decisions." What kind of decisions? "Feature decisions, customer-support decisions, technical decisions..." How can you delegate each of those? "Well, I can make Jill my lead engineer and direct technical..." You get it. Actually, just read this: http://sivers.org/delegate. You probably think only you can make the decisions. You're probably wrong and people will surprise you with their ability to take this stuff on, especially with your guidance. And if they can't, fire them (seriously, this isn't optional).
2. Operationalization. With delegation comes operationalization. You always have to be careful with process, but you also need to create it. When a new problem comes up, don't just solve it. Create a process for solving it and then apply it. This doesn't have to be the all-singing, all-dancing Ultimate Process (TM); it just has to be a way to solve this particular problem for the next, say, couple of months. "How do we structure customer meetings?" "When do we give them updates?" "How do we structure projects?" What's nice about process is that it applies DRY to your thinking; the process guides your (and everyone else's) decisions so your brain doesn't have to.
3. Brutality. This is the hard one. A small company needs focus. Focus is at, like, the top of the list for attributes you're after. So you need to look carefully at the portfolio of stuff you're doing and ask, "Do we really need to be doing this? Is it essential to our existence?" You (and whoever else really has to be in this conversation) need to answer it without blinking. It's hard to turn down clients, or cool ideas, or whatever, but you have to, because there will always, always be more stuff to do than you can plausibly handle. So you have to cut out all of that stuff, and you need to do it brutally and unflinchingly. "We're not doing that." Once you kill all the extraneous projects, even the ones that are making you money or publicity or whatever, you'll feel so much better.
4. Expectation-setting. Your developers want to work really hard, but that's actually not as great as you think it is. There's a tendency for employees in a small company to match the culture around them, especially when those employees are young. (I'm pretty sure that's where my twenties went.) But it doesn't make them magical animals; they'll burn out too. It's your job to create the environment of sustainability. That starts with your client relationships and moves down into your development schedule, your individual expectations for work hours, etc. You need to make the expectations explicit. Never ever (ever!) say "whatever it takes" and don't allow your employees to say; say, "here's what we can do", where "what we can do" fits into a reasonable schedule with reasonable hours. Will you lose some deals? Of course.
5. Planning. "um we have 8 pages left to design for tomorrow, nobody is going anywhere" is the opposite of planning. How did you get here? Well, you took on work without planning for it. That's going to cause you all kinds of stress, because it's crazy and humans can't work that way. Planning would have allowed you to say, like two weeks ago, "well, we're not going to make that deadline, so let's talk to the client and push that out." But instead, it's the day before it's due and you're fucked. So the first thing you do, on Tuesday when you're back is get a nice solid list of your existing commitments and see if they fit in a schedule. They won't, so then you'll have to reorganize some stuff and have some tough conversations about what to cut or push back or outsource or whatever. But holy shit will you feel better when you have. Then create a planning process going forward.
6. Supply and demand. Do you have too many clients for you to handle? Charge more. Seriously. The work will select itself for more value for you. If people keep wanting to pay, keep raising your price until they don't. You'll have less work and at least as much money.
Whatever you do, don't say, "let's just get through this last couple of things and then we'll be in the clear". You should know by now that's wishful thinking, and wishful thinking is, frankly, what got you into this mess in the first place. You need to take responsibility now and reorganize your team and process for sustainability.