i updated my macbook about 2 weeks ago and ever since i have not been able to log into my main account. i can use guest mode, but not my real account. every time i try to log in it has a loading bar, then the screen flashes. after this it acts like its trying to update again, it can get to about 10 minutes then says, "error could not install update." after that i restart it and goes to login screen again. after that its a continuous loop. any help? :))