One way is to match up photos with hard proof of a date. Like a holiday picture somewhere of the two of you and flight tickets or hotel receipts, as an example. Same with concerts, shows, etc. Get creative here. Should be easy to find receipts through email or bank/credit card statements.
A stat dec from the mother explaining the living situation is good.
What about dated mail to the address? Bank, government, work correspondence with your name and address and some with your partner's name and address? What address did the two of you file taxes under?
What about joint insurance (vehicle, contents, life, private health, etc)? Surely you can find an email confirmation of signing up for these or a dated letter? Even joint memberships like Flybuys or gym memberships or something.
Some people have used screenshots of facebook to show dates. As an example, my partner and I played in a social sports group where we lived, and the group often posted photos after games. We put in a screenshot of the group page that had a photo of us together after a game. Think holidays as well (Christmas, Hanukkah, New Years, etc) as that can show your relationship over time.
These are just a few examples off the top of my head. There have been quite a few threads discussing topics like this that you can pull ideas from as well.