When I boot I keep getting the screensaver asking me for my password.
I enter it, it is accepted, then after a few seconds the screensaver reappears asking me for my password again. The password is correct, because if I enter it wrongly it complains. The only thing I can do is reboot, but that doesn't solve the problem.
I understood that if I press SHIFT on booting I would be able to try an older kernel but holding SHIFT doesn't work on my HP laptop. It just boots normally.
I would appreciate some help on this. I'm not very experienced, so please keep it as simple as possible. Thanks!

The 'login loop' problem has been fairly common on recent Ubuntu derivatives so I would first suggest that you do an online search for 'login loop with Linux Mint' and you should get a number of hits such as those below, both of which are specific to XFCE. I posted the 2 links below as they are both marked SOLVED. Read through and if you try steps suggested, make careful note of the steps you took and the results so you can post the information here if necessary.

https://forums.linuxmint.com/viewtopic.php?t=261704

https://forums.linuxmint.com/viewtopic.php?t=278990

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Thaks yancek!
I had already tried searching online, but you gave me the right words to search for.
I tried No luck.
Then showed me that the root filesystem was full.
then got rid of a lot of stuff and left me with 81% in the root filesystem.

And I got my desktop back!