Stress is definitely a constant companion in the freelance world. Those who can't handle stress shouldn't even consider freelancing. That said, there needs to be a recognisable point at which useful stress crosses the boundaries into potentially harmful stress.
A little stress gives you that push to achieve - too much stress can push you into a hospital bed. Learn to recognise your body's signals, and if it all feels as if it's getting too much for you, seek medical help.