Division 2 Says I never purchased it even though I preordered it and played it for months...

I believe their was an error with the purchase servers that effected some people. It perhaps had an issue validating your licence.

Try a full console reboot and try again, the server problems have been fixed.

If you're not using gamesharing and you definitely own it yourself you can double check your account to make sure it's still appearing correctly

https://account.microsoft.com/billing/orders

If it is usually the two most reliable ways of fixing such an issue are:

1. Hard resetting your Xbox (hold power button down for 10 seconds)

2. Removing your profile from the Xbox and then signing in again:
https://support.xbox.com/en-US/my-account/manage-gamertag-and-profile/add-remove-account-xbox-one

When I try to start the game it says I don't own it when in fact I do. You can see my achievements, I have all the things downloaded, I have a leveled character etc.. on uplay I have the game and achievements etc... I have footage of myself playing
the game. Starting this month though I have not been able to play the game at all.

Maybe you were gamesharing with someone who had the game?

Make sure you are using the account that made the purchase and verify if you were gamesharing before this happened.

Please check this page if you can see on your account that you have completed the purchase for the game.

https://account.microsoft.com/billing/orders

If not, it means that you do not own the game.

If yes, do try to check with Xbox support for them to check your account game license.