I am so sorry to read about your loss. I know how devastating it is, and my heart goes out to you now.
I work at the front desk in a vet facility, and I have come to learn a little bit about causes of death, life-threatening conditions, etc. It sounds to me like your vet has a good handle on what caused Berrin's rapid decline. Just because he missed a dose of medication or changed the schedule on another doesn't mean that there wasn't also something sinister going on. The medication changes were probably coincidental and had nothing to do with why he declined. The stiffness in his legs preceded the medications, and is more likely to have been related to the cause of the problem,especially if your vet saw something that made her suspect this was neurological in nature. As for the lumps on his back, I know that if she had suspected malignancy, she would have recommended biopsies to rule that out.
It's always so hard to have to say farewell to our companions, and each of us recovers in our own time. My sister and my best friend from childhood both have to fill that void much sooner than I am ever ready to. Try to remember that our animals live life in the moment, savoring every experience to the fullest. They don't hold on to sorrow, anger, or loss the way we do. They always enjoy the best of times. We can learn a lot from them in that respect. It's perfectly normal for us to start off a relationship with a new friend with some level of expectation, including comparing them to our friends who have left our sides. in time, they develop their own routines, we begin to see their personalities shining through, and they take up a new place in our hearts. The ones who have gone ahead, remaining snuggled into their own spaces forever.
I wish you and your family continued healing and brighter days ahead. May your next dog clearly tell you when the time is right for you to find new love. Whether it's a week, a month, or a year from now, it will be the right time for you.