Why does face look different when I tie my hair up, in comparison to when I wear it down?

Casper: It is because with your hair up, it is putting upward pressure on your eyebrows and facial skin.  A ponytail acts a bit like a nonsurgical facelift and browlift.  I think one of your concerns is that you have somehow damaged or permanently changed your face by wearing your hair in a ponytail for 12 years, and that is simply not the case.  A browlift and facelift would make the same changes permanently and you would look even better than when your hair is up!  Best of luck. 

By tying hair up in a tight ponytail, you are actually
lifting the tissue above and below your ear, creating a lift. When hair is tied
up, you are likely to see a tightened brow, wider eyes and defined cheek bones.
However, when you wear your hair down, the lift is lost and facial laxity can
cause these features to appear less defined, and in some cases sag. Without
pictures it’s impossible for me to suggest a treatment or procedure, but be
assured that surgical intervention may not be required.

You  have discovered that a ponytail hair style gives you more of a lifted/youthful facial appearance.  When you were younger there wasn't much of a difference with hairstyle change.  As the face gets older, one gets more of a noticeable lift effect with wearing the hair pulled up.  Rest assured, wearing the hair in a ponytail for many years did not cause any accelerated aging of your face.