Hey AceSlayer03,
This would really depend on what you mean by merging your progression. Progress is stored on the EA account level, not on individual platforms like Xbox or Steam. This means if you link one of these platforms to a different EA account, you'll lose access to that content.
Cross Progression is just the process of letting you access the content on a specific EA account from any platforms connected to that same account. It does not enable the transfer of progress between different accounts.
If you played on Xbox through one EA account and then linked that Xbox profile to a second EA account, you'd be able to access whatever is on that second EA account - it would not copy the progress from the first EA account. If everything on your Xbox account was lost, it sounds like that may have been linked to a different EA account before getting moved where it is now.
What you can do is try re-linking your Xbox to whichever EA account it was connected to originally, or even just logging into that original EA account on PC to check what content may be available there. That's not always a guarantee as the process of unlinking can cause issues, but it would definitely be worth a try if you had progress that you're now missing.