Whenever we care for something, or somebody, there is pain waiting at the end of it.
That's the agreement we enter into when we first embark on a relationship with our pets, but that knowledge never ever eases the pain when the time comes to part.
You must remember the good times, and remember that although it would have been easier to let her hang on, and avoid the pain you now have, you loved her enough to do the right thing for her, and give her the peace she deserves, even though it hurt you so much to do it.
If she was able to talk, she would have thanked you for stopping her pain, for loving her enough to let her go, even though you wanted her to stay.
When the raw grief blunts a little over the days and months, that is the thought you must keep with you - you loved her enough to let her go, and you cannot love any pet more than that.