I know people here say Squats and lunges will solver your problem...
I am very sorry to say that I do them (many variations) 3 x week and they didn't save me from losing my bum.
And besides going flat, there were a lot of loose skin hanging there too

Well that was when I was weighting less than 120.
I got some weight back and therefore my bum did return too.
I would rather have no bum though.
My point is: Squats and lunges didn’t solve the problem for me, but I think could have been worse if I didn't have done them.
Maybe will be different with you...
BTW I lost my breasts too