Make sure the Caps Lock and Num Lock keys are activated and deactivated when necessary while you're typing your password. An accidental activation of one of these keys will invalidate your password, as passwords are case sensitive.
Review also your account activity to check for any suspicious activity. To do so, go to the Google account page, select Security and in "Recent security events panel" click on Review security events.
If you suspect something suspicious in your account activity, select No, it wasn't me and follow the instructions to secure your account.
If you keep forgetting your password, take the first letter of each word in a long sentence that you can easily remember and add upper and lower case letters, numbers and symbols. Don't use characters you might forget. However, your password should not be guessable by anyone and should have at least 12 characters long to be strong.