The subject of using one space or two between sentences, especially for people used to typing exclusively on typewriters or in following a style guide that is sometimes required, often creates colorful conflict.
Here’s the history: when monospace (fixed-pitch) fonts, like Courier, were in wide use, it was traditional to leave two spaces between the period and the start of the next sentence. There were definite rules for people who typed documents, but the rules have changed..
Today, we generally use proportional fonts (in which the width of the characters varies). Monospace fonts are more often used in databases, tables, etc.
Here’s the latest commonly accepted standard in the world of typography: choose one space as a rule unless two spaces are needed to create an adequate visual break between sentences.
There are exceptions. When the fonts are quite small (for example, a 9-10pt serif font like Times New Roman), using only one space after the period may not always provide a clear visual break between sentences. Generally a larger point size does provide that visual break. When in doubt, use your own judgment.
There’s always a way to work around your problems. And you’re never too old to learn. I’m living proof of that trite but true phrase.
Tip: if you’re tryng to use the new style, but don’t want to check one sentence at a time and you haven’t used periods (or other punctuation marks) for anything but ending sentences, try Search and Replace. Highlight your entire document, then go to Search. Type the period