Xbox live account associated with another account

It sounds to me like you have forgotten the Microsoft account associated with your xbox live account. When you have your Microsoft account associated with a Microsoft account it wont allow you to associate the Micorsoft account with another gamertag. Try
another email address that you might have used on the xbox live account or call support, give them your gamertag, Confirm that you are the owner of the account by answerring the security questions and they will be able to confirm the Microsoft account associated
with your xbox live account.

Not sure if this is related but I've tried to reset my xbox and as I add my account details it says that my microsoft account is associated to another profile of the xbox live, to try another account.

This may mean that the original Microsoft account or your Xbox Live account is in one of the following states:

•The Xbox Live account has a past-due balance.

•The current Microsoft account associated with the Xbox Live account is deleted or expired.

•The Xbox Live account has been flagged for unauthorized access.

•The Xbox Live account has a local currency balance.

•The Microsoft account you are trying to move your gamertag from is under 30 days old. (This helps prevent your gamertag from being stolen by an attacker.)

Or, it may mean that the “target” Microsoft account is ineligible because:

•That Microsoft account is associated with another Xbox Live account.

•That Microsoft account is associated with a billing account.

Unfortunately you cannot delete a gamertag. It can be moved to another Microsoft account or the account associated with it can be closed, but it will remain in Xbox LIVE's system.

Le compte Epic avec lequel je suis connecté utilise la même adresse mail que mon compte Xbox Live mais quand je veux associer ce compte Epic mon compte Xbox Live, on me dit que mon Compte Xbox Live est déj associé avec un autre compte Epic...

[Translation - The epic account I am logged on uses the same email address as my Xbox Live account but when I want to associate this epic account with my Xbox Live account, I am told that my Xbox Live account is already associated with another epic
account... ]