why cant i sign into my google account on my phone?

i have a droid turbo 2 and every time i try to log into my gmail account it just refreshes then asks me to sign in again. its been like this for a few months and its really a problem. without being able to sign into my gmail i cant download apps, i cant use the youtube app at all, and i just really need to fix it. ive been doing research on forums like this but i cant seem to find anyone having the same problem and i just dont know what to do. any feedback would be appreciated.