At 16 I decided that I was going to start going by Ms on my 18th birthday, as I would legally be an adult at that point and I associated the title Miss with children.
Twenty years ago when I turned 18 I followed through with that plan. After marriage I did not take my husband's name and so I still have the same name and title as when I was 18.
In my profession people tend to use first names in emails. However, when more formality is called for, I always address women as Ms (unless they're a Dr or something). Likewise, people address me and my female colleagues as Ms over email before switching to first names.
I find Miss and Mrs to be quite old-fashioned terms. I'm astonished at how many friends I have who use these titles.
I believe that if you do go down the Miss/Mrs route, the convention is that you remain a Miss if you keep your surname upon marriage. So Cherie Blair is Miss Booth or Mrs Blair, depending on whether she is being addressed in a professional or personal capacity.
I find Mx to be pointless and unpronounceable. Quite frankly, I'd rather be a Miss than a little Mx.
The history of Miss and Mrs is fairly benign as far as I can work out. Both titles are diminutives of the word Mistress, as in the Mistress of the House. The lady of the house was Mrs and her daughters were Miss - the equivalents of Mr and Master. However, as boys grew older they all became Mr, whereas girls only became Mrs upon marriage.
In other European countries the terms Fraulein and Frau, Signorina and Signora, Mademoiselle and Madame developed along similar lines to Master and Mr. It does seem odd that in English-speaking countries we've made such a hash of something so simple!