Hi Lily,
We totally understand how frustration this can be. We would like you to know that it is by design that you will not be able to use your old password after changing it. This is a security feature of all Microsoft accounts which cannot be turned off.
If the option for 72 days password expiration is enabled in your account then it will automatically ask you to reset password.
To disable this option you just have to make sure that the option “Make me change my password every 72 days” is unchecked while you reset your password.
And if you're signing in to a Microsoft account, your password is always limited to 16 characters.
For additional information please refer the link below.
https://www.microsoft.com/security/pc-security/password-checker.aspx
Hope this information is helpful. Please post back if you need any further assistance, we will be glad to help you.