I am so sorry.
I don't think there are words that will make this better. I went through this in September 09 when my sweet Cody passed from renal failure. All I can tell you is that I went through the same emotions that you are. One day I woke up and I knew it was time.....they do let us know. When it is time, your sweet Meg will let you know.
Sadly, these are things that we must experience in life. It is all a grieving process. I can tell you that it never goes away, but it does get easier. In time, the fond memories carry us through.
Again, I am just SO sorry.