I recommend taking a look at an introductory Historical Linguistics book. The answer isn't too complicated, but it's more complicated than what one can reasonably say in a Reddit comment. I'll give a summary of some key points though, but then I'd suggest checking out the library for the listed books at the end.

So, every living language is constantly changing. That's basically what we mean when we say "living" vs "dead". "Latin is a dead language" means no one's speaking it in their daily lives since childhood so it's somewhat frozen in time. The English you speak, however, is different from how your parents speak. The difference may be too small to notice without some specialist training, but it's still there. The word "apron" used to be "napron", and "a napron" shifted at some point into the past such that people started thinking "apron" was the word. Now we say "apron".

I understand it wasn't instant, but what caused this huge change?

So actually this is what happened in your example, but a dozen times over. You're right that it's not instant. Any single small change will happen over a generation. But they're usually tiny changes. It's the compounding of those changes that make it seem like one massive change, when really it's so slow that people don't even notice it for the most part.

Also you should keep in mind that asking "Where did the extra characters go?" doesn't really apply. Language is written down, but writing is not language. Your <þ> is just written <th> today. But <þes> ant <this> don't actually differ much in pronunciation.

Further reading:

• Theodora Bynon, Historical Linguistics (Cambridge University Press, 1977)

• Lyle Campbell. Historical Linguistics: An Introduction. (2013).

• Larry Trask & Robert McColl Millar. Trask's Historical Linguistics. (2007).

• A Linguistic History of English

A great book would be The Cambridge Encyclopedia of the English Language by David Crystal. We used it as our textbook for our History of the English language class, and I loved the layout, the articles and asides. Great resource. Thorough but fun.

You might be interested in the book Righting the Mother Tongue by David Wolman, which chronicles the changes to English orthography over time.

The short version: the Angles and the Saxons and various other tribes spread across the land that would become England, each with their own Germanic dialect. Basically each town had its own language. In the 9th century, Alfred the Great unified a fair amount of the region, and his dialect of Germanic became more widespread, becoming what we now consider Old English. Then, when William the Conqueror took control of England in 1066, he started conducting official business in French, and from that, English got an infusion of French words. This became Middle English. When the printing press became widespread in the 15th century, it typically only came with the Latin alphabet, because it was often used to print bibles (in Latin). Consequently, most printing presses didn't have edths or thorns or yoghs, and the typesetters had to improvise and come up with ways to represent these sounds using Latin characters. This became modern English. The spread of printing presses also started to standardize spelling, and the modern dictionary didn't really exist in English until Samuel Johnson made one in the 18th century (French got a standard dictionary in the 17th century from the Académie française, but before that dictionaries tended to be shorter lists of unusual/specialized words, rather than the comprehensive tomes we know today). Then, after the American revolution, people like Noah Webster decided that although America had gained its political independence from Britain, it needed more of a linguistic independence. His efforts (including his dictionary) are where the theatre/theater and colour/color changes came from. There have been various efforts to simplify and regularize spelling since then (such as the efforts by Melvil Dewey, of Decimal System fame), but they haven't been very successful. Nonetheless, the language continues to evolve today, with new phrases often popping up on accident, because reasons.