Had to euthanize my cat yesterday 😞

I feel bad. I had a mission to save her when my husband found her as a stray. She was thin and alone. We brought her home 3.5 years ago and got her to gain weight and she looked really good.

After we moved into our house she looked great, was affectionate and loving. The sweetest cat. I think her downward spiral began when she would throw up her food. I thought she was just eating it too fast. The past 10-14 days is when it got far worse. She wasn't grooming. Her appetite decreased and she began loosing weight. Since I have helped her gain weight before and nursed her back to health I thought I could do that again. Yesterday morning she was walking and dizzy. So I got a towel and laid her on my bed. She wouldnt move hardly at all. She was interested in the food I offered but she wouldnt touch it. She had what I thought was a seizure since her body was convulsing and her eyes twitching. I thought then we were loosing her. My husband and I took her to the er vet. There they ran xrays and found a 4cm mass on her stomach. Believed it to be lymphoma which is common for cats. The seizure the vet said could be the cancer spread to her brain. Because she had lost so much weight the first battle would be to get her back to stable before even risking surgery. A week hospital stay and there is the risk she could still pass on her own. Too much iv fluids could shut her system down. We made the difficult decision to put her to rest. Since the outcome and her condition didnt seem best for her. I couldnt see her suffering. As the vet started to give her the fluid but it didnt reach her yet, she stood up. This has me thinking did we do the right thing. Is there something I could have done differently? I have no idea her age either since she was a rescue. If I had to guess id say over 10. I dont know how long she spent outdoors before she came with us either. All I know is we loved her and kept her from spending another cold lonely night outside.