Please help! We are planning to go to Cancun, and won’t need a test on arrival back to US since we tested positive within the last 90 days. Could someone please share their experience if you were in the same situation? Thank you!!!

Hotels are reportedly offering tests so it would seem the answer is just get one.

And hope you don't test positive again. Very low odds of that, but it's entirely possible...

https://www.usatoday.com/story/trave...ts/6982341002/

The answer may be to get vaccinated as official talk of a "covid passport" will eventually yield a common solution for everyone before too long. Though there's a note of desperation in your "Please help!" I have little sympathy for those who treat not traveling as a life-threatening condition when the real one can be travel itself. We're all chafing at the bit & Cancun will still be there when the solution arrives.

I would read Jean's link.
I would follow MmePerdu and wait on taking this trip. Not worth the risk of getting stuck in quarantine in MX - for me, anyway.