I had a lovely GSL named Ariel (after the mermaid; yes, I have a young daughter). She was one of the most delightful chickens I ever had. She would follow me around, sit in my lap, allow me to just pet her, eat out of my hand...delightful in every way. When she was about 2 years old she stopped laying. Within 6 months I realized she had a problem and I knew she was laying internally. I also knew there was nothing to be done about it unless I wanted to start tapping her belly to drain off the fluid that was rapidly collecting there. I didn't want to aggressively treat her because I knew she would stop trusting me and start to look at me as though I was her enemy. She was my best chicken buddy right up to the day I decided to break her neck and put her out of her misery. She was pretty far gone at that point, anyways. I probably should have done it sooner. She was one of the only chickens I ever cried over. And I will never have GSLs again because they are so sweet, but so prone to EYP and internal laying.
I am so sorry for the situation you find yourself in. We all understand here. Most of us have been there ourselves. Take comfort in the fact that you are doing the right thing by her.
Good luck.