how do i activate an email address i changed

I created a new Xfinity email address two days ago. Originally I could switch from old to new and had the new marked as preferred. Today, The old address is not listed on the email screen for me to change it back to preferred. I realized I am not receiving emails on new Xfinity address. I tried to log into old address and I am told it is incorrect ID. I tried to change it back to the old address, system tells me it is associated with another account.