My heart breaks for you, OP.
My two loves would have been 19 this year, but I lost them six months apart at the height of lockdown/2020.
I can't take away your pain, but maybe I can make you feel a little less alone.
My one dog had been very ill most of his life. Ironically, he lived longer/was the older one. My younger dog unexpectedly died at age 13. Like yours, they noticed the pale gums. There had been something "off" with him for two weeks and we couldn't figure it out. Long story short, a trip to the ER determined that there was bleeding in his chest due to a tumor and that the risk of interference was far too high and the right thing to do was to put him to sleep then and there. The guilt that I felt afterward was crippling. It was somehow easier to know when the time had come for my other little guy as he had so many health issues for so long.
Here is what I can tell you. I cried for six months for each of them. I still cry at times, and it has been almost five years, but more often I now smile. I know that they were loved, and that grief is love. I know they felt that love and I believe that they are still here with me. Now I can smile looking back on those memories because each one is a gift, just like they were.
I won't sit here and tell you that 19 years is an amazing amount of time (it is, though) because no amount of time is ever enough. That was your baby.
In the end, you have to give yourself the grace of knowing that letting go was your final act of love. You wouldn't have wanted your sweet little dog to suffer. And when a dr says it's time, they usually mean it, because they love animals AND know when they can be helped and when they cannot.
Take all the time you need. It's different for everyone.
For what it's worth, I dared to open my heart again, and have been blessed with two more sweet dogs. I never thought I'd love like this again, but oh, I do! AND....I know that each of my dogs sent these two to me. I really feel their spirit in them.
Your little dog will always be with you. Just in a different way now.
Hugs. I now all too well how deep this pain is. I also know how lonely it can be when others don't understand or show up for you in this moment.