Some apps like FB/whatsapp will save (manythings) only to internal storage. You cannot change it. Either move photos/vides from whatsapp to your SD card or in the file manager look for a folder called Android (just delete it). Every app keep lots of rubbish inside it.
In the worst case copy all your photos/videos to Google or computer. Go to Accounts->Remove account->google account. THEN. Reset the phone. A clean reset is the best.
Do not reset before removing google account. lets us know if all is then OK.