Very happy for you that you had that moment - I know how it feels! I had to deal with many, many difficult clients throughout the years. I got a lot better over time, and managed some really, really difficult ones over time. My girlfriend works in direct contact to her clients, too, and we have a lot of clients in common. It's fun to tell each other how shitty some of them treat us, takes some of the edge of.
I try to teach that way of handling difficult clients with professionalism and kindness, even though they annoy the f out of me or my proteges. It helps to realize that everyone lives in their own little hell, and that all we can about others is being kind and staying on our course.
I thank a lot. All the time. I prefer being too thankful (and feel it, not just say it), than missing out. As company founder, I have to be quite harsh and demanding sometimes, so I really try hard to even that out by recognizing progress and jobs well done. It's also very important to me that people express it, a part of good manners, a base of communication.
And yes, I have received a lot of gratitude over the years. But I can't recall a story like yours, that someone very difficult came out so positively. You really must have done an outstanding job, congratulations!
People often forget how important person to person interaction can be in the moment and for future occasions and, of course, manners in general seem to be in decline across the board. It doesn't take long or any real effort to show some gratitude but the benefit that can come from it is huge.
People have their own scenarios to work around and they sometimes exert pressure which can cause them to focus on their own needs rather than others, and therefore bring less than optimal performance/manners etc. it's a shame really, but it's a human thing I guess.