[Westbrook]

Save your data.

Get the latest mint image.

Reinstall with your new partition scheme.

As for your password, boot in single user mode to get to the root account.

Change your user password from there.

This is why Linux users should encrypt their boot disk.

But if you had, then you’d be hosed.

[djf]

Save your data...
Well that’s what I’m trying to do - I installed a new copy of Mint and was copying from the old partition to the new partition, ran out of space,
How does “single image mode” show up in Grub?
BTW, I’ve been using Linux RHEL 8,1 for my work computer so I’m not a Linux newbie,

[Bloody Sam Roberts]

Ditto. Take monthly...or even weekly images using something like Redo Backup. That's if something went sideways. You can re-image your HDD or a new larger one. Then boot in single user mode to get to the root account.
Then use GParted to resize the partition Linux is on.