If you do not have root password set in Linux Mint, and you have managed to forget your user password, you can recover your system by setting root password for your system, here is how to do it.
Reboot your Linux installation, and press and hold the "Shift" button on your keyboard.
In the GRUB menu select "Advanced Options for Linux Mint" and press Enter
on the next screen select the (recovery option) and press Enter
You will have to choose the "root - Drop to root shell prompt" option
if you press Enter here you'll get to the root shell prompt. Because we need write permissions here, let's re mount our system root folder with the following command :
mount -n -o remount, rw /
The next step is to change our "root" password by using the following command :
Make sure you are using a secure password for your root account as it's extremely important to keep this account secure and well protected.
Now re-boot and will be able to use your system with your new "root" password.
To restartyour system, simply type this command in your terminal and press Enter