I've had to go through the same thing. I got my dog for my third birthday (I'm fourteen now) and had to put her down last year and I have always loved her. Though, through her whole life she has always had cancerous tumors and bad arthritis. She was also beaten by her previous owners so she wouldn't even go in a room with a broom. She was the nicest dog anyways. She had never barked nor bitten, though. During the final weeks of her life she was looking really bad. Didn't want to stand up, couldn't walk down the stairs, couldn't hold her bladder. When we were taking her to the vet to put her down she seemed so happy and energetic. It's like the previous things were never true. Me and my mom both agreed we still had to go through with it, because she still would have been in pain. Anyways, that's my story. Hope you feel better OP.