My 2 year old pitbull died from lymphoma and I’m still seeking closure

Like the title says my 2 year old pitbull named Prudence died of lymphoma 6 years ago and it was probably the hardest thing I’ve ever experienced to this day. Ive always wondered what happened and why this awful disease chose her. She died a month short of her 3rd birthday and lived with the disease for 8 months. I tried so many different treatments.. we were in and out of the vet continuously for spiked fevers and various other ailments. I truly believed that my dog was going to be one of these miracle success stories and she would be healed but I was wrong. I get mad sometimes that I didn’t just put her down because it would have saved a lot of heartache and pain. She had a really great last day but unfortunately I was spending the day with some jerk guy.🙁. I still get mad at myself but I know it’s all irrelevant now. I just wish things were different!! She was the most amazing animal/person and we had a bond like none other. I raised her on a bottle from 2 weeks old and she became soooo much of my life. It just makes me sad and I’m ranting about it now. More than anything I’m curious what caused this disease? Has anyone else had young dogs diagnosed with this? It feels like it’s always been my fault she died prematurely.