If you’ve forgotten your Steam username and password, go here to try and find your account - https://help.steampowered.com/en/#HelpWithLoginInfo
If you are unable to complete the account recovery process, you will need to try and verify your ownership of the account here - https://support.steampowered.com/kb_article.php?ref=5421-QTFB-3090
Please bear in mind Sports Interactive and SEGA have no access to Steam accounts and password and cannot help in this situation. You must use Steam’s support in order to get support with this issue.
Edited by Neil Brock