Don't feel bad. You are doing the best you can. It is hard to accept that time sometimes. All of this is hard to accept sometimes. I am with Marycain, maybe you could change to a less stressful position, for just a little while. It doesn't make you less of a nurse, just makes you smart about trying to manage your health and your situation. And as long as you are spending some time with your kids, and trying to make it quality time within your energy limits, your kids should understand. Mine are 10 and 11, and they have been amazingly resilient.
Hang in there. Don't be afraid to vent to us and don't feel like a jerk! We are here for you!
Take care of yourself.
"If you think you can, you can. If you think you can't, you're right". - Mary Kay Ash