Scared my dog didn’t understand what was happening

We just put down our Newfoundland today, she was going going to be 7 in June. She had arthritis and instability in her back legs that was manageable but recently developed very aggressive bone cancer in her front leg. Now she could barely walk even with her pain and inflammation medication. She couldnt even get outside to go to the bathroom by herself and she would refuse to just go inside. Really all she could do was lay around and cry for help when she wants things or get up when she really needs to. But she was still mentally all there which made it so hard. She wanted to do everything so badly still, she just didn’t understand not being able to. Today the vet came to our home to put her down and she got so excited by visitors that she perked right up and limped over to greet them wagging her tail and it broke my heart. I’ve only had an 18 year old elderly cat with lung cancer be put down before and he clearly knew he was on his deathbed but I don’t know if she knew and that makes me feel so bad. I wanted to say no so badly after seeing her perk up. I know it was the right decision, the cancer was growing very aggressively and she was in pain and had a poor quality of life but it feels so bad when it feels like you’re making the decision. I’m scared she didn’t know it was over for her. She wanted to live longer so badly.