You didn't tell us what the computer store's diagnosis was. That would be helpful.

You say the cursor jumps all over the place. Is that even when you are not touching the mouse, touchpad or keyboard?

Try booting Windows 10 into Safe Mode and see if it behaves any differently.
https://www.digitalcitizen.life/4-ways-boot-safe-mode-windows-10/

At 5 or 6 years old, the hard drive may be failing, or there may be other hardware problems. Do you have the skill and or desire to disassemble the laptop to get the hard drive out? Do you have a SATA to USB adapter cable or dock to hook the hard drive to your computer to try to get data?