Okay, don't panic.
The good thing is that you only have to prove the last 2 years of living together, not the full 2.5 years, and looking at your previous posts it looks like your visa started on 27th November 2016, which means it will expire around 27th August 2019.
So, that means that if you wait until August 2019 to apply for FLR(M), you only need to prove living together from August 2017 to August 2019... which gives you 1 year and 5 months to collect as much mail as you can, and only 7 previous months to try to cover (August 2017 up to now).
You only need to provide 6 documents in each name total - so that's 3 per year.
You'll need one in each name (either joint or separately addressed) from approximately the following months (but you can adjust the months accordingly depending on what you have):
August 2017
December 2017
April 2018
August 2018
December 2018
April 2019
(Extra one: August 2019)
If they are separately addressed, then you'll want them to be dated as close to each other as possible (i.e. from the same month, ideally within a few days of each other).
Best thing to do ASAP is get your names on as many items of mail as you can (bills, utilities, bank statements etc.) and start collecting them. Also, TURN OFF paperless on EVERYTHING.
Then you just have to see if you can dig up anything you can use from the last 7 months.
Do you get any of your US bank statements sent to your UK address? You could use those if you do.