Some time when I was younger I linked my account under a different minecraft username (still the same account today ) but I have tried to link my account now under my new username (I have changed it multiple times from then to now) and I can't find any way to unlink an account because when I copy the link account command from my account settings it says in chat that I have already Linked my account and have no idea how to fix this problem.

you'll have to email support@swdteam.com to get the account unlinked :)