In the old days of typewriters using only monospaced fonts in which a period occupied as much horizontal space as any other letter, it was standard to double-space after each one to clearly separate each sentence from the following one. However, when justified variable-width type is set for printing, it has has been standard since the mid-20th century to use only one space between sentences. Modern computers produce type that is more like print, and most modern styles call for only one space after a period. This is especially important if you are preparing a text for publication which will be laid out from your electronic copy. If you find it difficult to adopt the one-space pattern, when you are finished writing you can do a global search-and-replace to find all double spaces and replace them with single spaces.