Good evening JennieJang,
Welcome to the Apple Support Communities!
I understand you are having issues logging into your Mac. You mentioned that you've changed your password several times, and I'm wondering if you are confusing your Apple ID password with your computer user account admin password.
So, I would first check to make sure your Apple ID and password is working and you can sign in to your Apple ID account page.
Then, reset your computer user account login by using your Apple ID.
macOS Sierra: Reset a login password
If you associated your user account with your Apple ID, you can use your Apple ID to reset your login password.
Choose Apple menu > Restart, or press the Power button on your computer and then click Restart.
Click your user account, click the question mark in the password field, then click the arrow next to “reset it using your Apple ID.”
Enter an Apple ID and password, then click Next.
Follow the instructions to reset your login password.