Hi, what a sad and tragic loss. A pet you grow up with, who shares those teenage years when no one else seems to understand is very special. You will feel deep loss and grief. Please don't let guilt rob you of the chance to grieve and mourn Max in the way you need to. Or to remember 12 and a half years of happiness and love. Sadly all of Max's symptoms lead to the same sad decision. Having a biopsy would have caused him unnecessary distress and I'm sure you wouldn't want that memory. You released him from suffering and for that you should take comfort. It is one of the few things our beloved pets ask of us in return for all they give. They need us to make the hardest decision for them. As much as it hurts, you did the right thing.
I'm sorry you have lost Max but he is around in spirit and will love you for taking his pain away.
Love you forever and ever and I will be there for you xx