I am--I have custody of my grandsons, ages 10 and 13. They've lived with me for most of their lives, but it was only about 3 yrs ago that I finally got legal custody.
Like you, I love them to death and would do anything for them, but, at times it gets tiring--esp when they start fighting (the 13 yr old just loves to pick on the 10 yr old). Then I want to string one or both of them up!
I, too, work full time. In addition, I was back in college until December (Finally finished! YAY!).
Yes, it is difficult at times, but it's something we have to do, isn't it?