Dani Mathers -- Dear Playmates ... Please Forgive Me!

Dani Mathers is in full-on damage control mode ... firing off an apology to her fellow Playboy Playmates.

The 2015 Playmate of the Year sent the bunnies an apology for her "thoughtless and mean-spirited" Snapchat post that cost her a radio gig and her LA Fitness gym membership.

In the email, obtained by TMZ, Dani calls posting a photo of an unwitting woman in a gym sauna a mistake ... apologizing for "the negativity and hate that this has brought forward." She also says there's no excuse for her behavior and she's laying low.

She never explains WHY she's doing a mea culpa to the Playmates, but it's a good bet she's afraid Playboy might yank her title ... so she's throwing herself on the sword.