Gmail does not use the website password on third party email apps. You need to set account for OAUTH2, clear out the password and Google should then prompt you to set up Thunderbird for your access.

how do i set account for OAUTH2?

i tried googling to find help, but didn't find instructions i could understand. why do we need oath2? we have been using TB to get gmail for years without any problem until today.

This is a gmail thing. To use gmail, you must change your account security setting and also remove your old password and then Google should prompt you to authorize access. This has been going on for some time, but Gmail's cutoff was somewhere in late May.

ok, i stumbled through it, but now i have 2 accounts in TB with the same name, both pop3. they seem to be both working. can i just delete the new one since the old one has the old emails and folders?

Yes, I'm glad you fixed it.