There is not much I can add to what Elaine has already said. I am so sorry. I do know what you're going through. I had been in this situation a few times over the years. I believe that God will guide you in doing the right thing. He has shown me several times that the only compassionate thing I can to is to let my beloved kitties come home to Him.
I lost my big Maine Coon boy this past April. Like Annie, he was fine one day and a mass was discovered the next day. That in itself is a shock. I have always told my kitties' vet that if they see me hanging on too long, to please tell me. My Zach suffered miserably towards the end (a very short period of time). I knew I had to let him go, to free him of his suffering. I also know that we will be together again one day in Paradise. In the meantime, he is very close by--right in my heart forever.
For the first time, and I should have thought of this sooner, I asked the vet to come to our house. Zach, the vet and I were in my bedroom with the door closed (other cats outside the door) and I held him in my lap and watched her give him the injection. I wanted my voice and loving touch to be the last things experienced on earth. I cried pretty much the entire time. My tears fell to his fur and I buried my face in it. I tell him all the time how much I love and miss him. I know he hears me. He has shed his old, tired, painful body and is totally free now.
Perhaps if you think of letting him go in this way, it will help i.e. you are releasing him from his pain and suffering and he will be in his true home instantly and that you will follow one day.
I was present when my Mom died. I held her hand and whispered to her in ear (I talked to her several minutes with my lips to her ear as I knew she was leaving). The last I said to her, when I saw the heart monitor flat line was "I'm going to let go of your hand now Mom. Jesus is here to take you home." I miss my Mom and my kitties every second of every day. I hate to think what it would be like for me today had I not done things the way I did both for Zach (and other kitties) and my Mom.
God bless you and Annie. You will be guided in making your decision. If you decide to let her go, try to find some peace in knowing that you are turning her over to her creator. Aside from being with you, there is no better place than she could be.
I will pray for you and Annie.