You're well out of the 'easy' part of it, most people associate the first year of a start-up with stress, but that's because they never get to the part where you're successful.

One major source of stress is family stuff, I don't know anything about your family situation, but keep your spouse, if you have one, in the loop so they can anticipate when you need them.

It also helps to really learn how to delegate, and do it well.

Physical exercise has been mentioned a couple of times already, let me add my vote to that bit, it really matters that you are healthy in both mind and body.

Make sure you don't take your work home with you, work hard but don't kill yourself doing it and keep in mind that there is more to life than work.

best of luck! Looks like you are well on your way.