Welcome to Android Central! After you reset and then set the phone back up again, is the phone automatically reinstalling your previous apps? Remember that an 8 GB phone only has about 3 GB or so available to the user, since about 5 GB are used up by the OS and any bloatware. If you're reinstalling apps after the reset, that can quickly use up the small amount of storage you have.
Do yourself a favor and upgrade to a more modern phone with at least 32 GB of internal storage. There are plenty of inexpensive options.