Forgot Username but also have no acces to grub

I would assume that your computer is using grub, as it is the default bootloader for Ubuntu. But, if it's not, or you can't use recovery mode for some other reason, try this. Boot off of the live flash drive, and select "Try Ubuntu" instead of install. Mount your usual boot volume to /mnt/main, or any other mount point. Use

less /mnt/main/etc/passwd

to show a list of users. Find your username, and exit less.

Then run

sudo chroot /mnt/main

This will effectively put you in a shell on your usual boot volume. From there, you can run

passwd username

and enter a new password. Exit the chrooted shell, and reboot without the flash drive. Login using the username found in /etc/passwd and the password you chose previously.