Can't Login to Macbook. Please Help!

I can't login to my macbook air (mid 2011) running macos Sierra.

What I have tried:

1. Recovery Mode with Command+R then I change password to new one. BUT, when I type that new password, it won't login. Although it worked when I used that new password to unlock the Disk.
2. Reinstalling the OS twice. But, got the same result. It won't login. Though I know for sure the password is right. (from the beginning I confuse why the password won't work, since I never change my password)

I don't know what to do now. Please, help me!

I'm desperate here...