@mesch and @everyone else who took the time to clarify this concept for me--thank you. I may be a little bit dyslectic when it comes to this stuff--the way I was seeing it was that with a 20A outlet & 12-2 connecting it to the box, I had a 20A circuit protected by a 15A breaker. When actually I had the opposite. But now I understand the error in my thinking & I am rectifying it. I do not want a catastrophic event--period; but if one occurs, I do not want my insurance company finding a way not to pay me.
Anyway, I would have already put those 20A breakers in, but since this is the dedicated circuit thread, something else had been gnawing at me. My circuit needed to move to another room a few years ago, so what I did was put a work-box in my attic for a junction from which I could drop the lines down into my new location. I went to Lowes to load up on the stuff I would need, and the electrician guy who was there sold me on these (for lack of knowing what they are really called I will call them) "stab connectors" instead of wire nuts. (one stab for black from box/one stab for black to new location/one stab for black going to original location X white & copper X three circuits . . . I never really liked the way I felt about the way they grabbed on to the stripped leads from the Romex, and a while back ago I wound up putting wire nuts on all the grounds because I lost a ground on one circuit (must not have been a patent stab) and after I read a reply on this thread about circuits made up of "back stabbed outlets," I decided to get up in the attic today and pig tail and wire nut ALL the splices on ALL three of the circuits. Which is not as much fun as it sounds like, because I had to reroute some of the Romex as I had it stapled down after I finished it the last time, and I needed more Romex in order to work with it inside the work-box. So that’s what I have been doing today & will finish up tomorrow.
So going back to a few years ago, when I was buying stuff at Lowes, the electrician guy who was assisting me asked me to describe the Romex I had used, & when I did, he told me it was only rated for 15A. Since I only do this when I need to, and since I originally did it 25 years ago, I thought it would have been strange that everyone who told me what I needed to do and the electrician who actually hooked my Romex runs up to the upgraded box I bought, would have told me to buy 15A Romex and then hooked it up to the 20A breakers I had for those 3 circuits, but I am not rocket scientist or brain surgeon so I decided to put 15A breakers in for those 3 circuits. As I was working on my new lines, I did note that my existing Romex was clearly marked "12-2 with ground" and I did later show a piece to an electrician who assured me that it was good for 20A. But, a 15A CB just seemed like an error on the side of safety for all the misguided reasons that I was asking about in my last half dozen posts or so to this thread, so I never asked anybody, until now, about the legitimacy or wisdom of that.
However, I am glad I did. Sorry for writing War And Peace.