I wanted to respond to this line. I don't discount your experience with this one dog, but as someone who previously owned a fearful and reactive border collie, I've gotta say that this line of advice is not at all helpful.
I brought my old dog Buddy home at 2-3 years of age, knowing nothing about his issues or about reactivity in general. He was terrified at the shelter, and beautiful, and I felt bad for him, so I adopted him. And BAM! What a surprise.
I walked into our first few weeks with blithe confidence: I'd owned many dogs, and none of them had had issues. I treated Buddy like a "normal" dog - and watched as he attacked other dogs I let approach, and lunged and growled at other people we passed.
I didn't enter into our relationship with fear and trepidation - I had total confidence. And after a couple weeks, I GAINED fear and trepidation, based on the very real scenes I had witnessed. Of course I was nervous when strangers insisted, "My dog is friendly!" I knew my dog was going to attack theirs, and that I was going to be in the middle of a dog fight (3 stitches and 3 hours in the ER for me thanks to a "friendly" dog, 2 paid emergency vet visits for other dogs who ended up with punctures).
The most comforting thing anyone ever said to me came from my trainer, who had also owned a reactive dog: "Until you've owned a dog like this, you have no idea what it's like."
With years of work, Buddy led a relatively "normal" life. I could never let him loose with strange dogs; he could never relax and play with more than one dog at a time. He eventually stopped lunging and growling at strangers and would even let them approach. And during all the time he acted "normal," I still had my internal fear and trepidation running; I never stopped being on guard and watchful with him.
But, oh, the countless people who 'knew better" than I did about how to work with my dog. The guy with the pit bull, who couldn't control him on leash, and who insisted that we had to just "let the dogs work it out; my guy's a lovebug!" as the two dogs postured to do death battle. That annoying woman I would meet in the woods, who would literally bend down and put her face in Buddy's face, to try to "teach me" that you just have to "push through" the dog's fear. (Honestly, if anyone was going to end Buddy's life, it was going to be her, after her bit her face off!)
Now I have a new dog. Ten years of background with Buddy influenced the way I entered interactions between Cricket and the world. I walked her with fear and trepidation early on, not knowing how she was going to react, and wired to expect behaviors from her that I saw from Buddy. And guess what? SHE ACTED LIKE A NORMAL, HAPPYISH DOG. She's simply not wired like Buddy, and things don't ruffle her.
Fear and trepidation are normal responses to dogs who display extreme behavior - often the result of the extreme behavior, and not the cause of it. I wish people in the world would be more cautious about finger-pointing at owners who are trying their very best to deal with the cards they were dealt when they took on a challenging dog.