There was a time when it was standard to put two space characters after each sentence before beginning the next. This was how I was taught to type, and I’m only in my early-40s. However, this is no longer the practice. The introduction of modern typesetting and justification algorithms mean that two spaces can result in gaps that are too large between sentences. Industry professionals now specify that we use only one space between sentences.
Retraining the brain to do this has taken me many long years, but there is a simple solution that will work every time, even when you forget. Run a search-and-replace, hunting out all the places where you use two space characters, and replace them with only one space character. All word-processing programs have the search-and-replace feature, so why not use it. (I do this as a matter of habit before I send a manuscript out the door.)