I went through this over the last year. After being told our 14 yr old lab only had a couple weeks, we brought her home and just enjoyed as much of our time with her as we could. That day, I gave myself a meter as to when I would make the call to the vet to do an in-home euthanasia (super super highly recommended; it made it easier on both of us). The day Signal stopped eating (my meter) & subsequently lost the ability to even stand, I had to make the call. It took me hours to get up the nerve to do so (what if she bounced back again tomorrow?). I loved that dog so much. My advice for Giz is that you should give yourself an absolute line in the sand as to when you'll need to make the call. I really recommend in-home services if you can afford them and if they're available in your area. It's disturbing how pleasant that made this terrible situation.

As far as the tumor goes, I had this issue with Signal. I just coated the wound in Neosporin and bandaged it daily. It got better. She also got pretty bad about peeing in the house, but adapted to using pee pads.

However this goes, it sounds like it was a great long life. Take solace in that.