What to do?

My girlfriend and I have been talking and dont know what to do. alot of the time when were not together and were texting eachother she ends up texting me things like shes not good enough and why are we even doing this anyways and, " you shouldn't date me you deserve better". and I dont know what to do. Everytime she says these things i ask her why she thinks these and i try and make her understand that i love her and appreciate her and that she is an amazing girlfriend, but she never belives me. we both are very bonded to eachother and love eachother very much but what do we do