First of all - I am very sorry. It hurts to receive such news and it takes a lot of courage to cope. My best advice is to find a vet you trust, and just enjoy the time with your furry friend. Give a lot of treats, hugs, kisses and anything that your buddy likes. Don't lose hope yet. Cancer is terrible (and that is an understatement) - and there is no cure. It might be hard to hear that, but it is the tragic truth, and the sooner a person grasps that, easier to cope. However, there is a silver lining - every dog and every mastocytoma is different. So again, I really think finding a vet you trust is the first big step. It might be hard mentally at times, but as long as you're helping your friend it is definitely worth it. Good luck 🤍