It sounds like you have two accounts now. You can't purchase without logging in or creating an account. So I think you will have quickly made yourself an account back when you started buying books. The books you bought are linked to that first account. If you now sync with your new account, your books will disappear as your new library (attached to your new account) will be empty. But don't worry, you can sign in again with your old account and all your books will still be there.
So, either you'll have to ask Kobo to merge your two accounts (if that's even possible, I don't know), or find out what your old account is (ie. which e-mail address you used for that) and log in with that. The latter I think is the easiest option for you: logging into Desktop with the old account will make all your books magically appear in the Desktop app. Your details in the Kobo app on your Sony tablet should say somewhere which email address you used to be logged in there, I think.