Start by finding where the biggest disk usage is coming from using this command, then narrow down the folder at the end:
du -s -h /*
Learn more here: https://www.redhat.com/sysadmin/du-command-options
It’s often those stupid binlogs, found at /var/lib/mysql/
You really don’t need them.
If it’s the binlogs, disable binlogging at /etc/mysql/mysql.conf.d/mysqld.cnf (I think), then delete the binlogs, and clear those files from the binlog.index. I usually save one.