I put my cat to sleep last March. I had him since I was 8, so he was mine for almost 15 years. It was one of the hardest things I'd ever had to do. I'm so sorry that you have to go through it.
The only thing I can say is that it does stop hurting after awhile. I know that may sound silly, but it's true. It's gonna hurt for some time....but the days are gonna pass, and you'll adjust to life without her. I'm really sorry to hear that though. Pets can be like family at times, especially if you've had them for years.