Because she had this perfect image of him. She thought he's no capable of these things.

Because when she found out about Stefan's she was sired to Damon so she really didn't feel for Stefan and when Damon did it she wasn't sired so she felt it big time

i would say she was more hurt when stefen slept with Rebekah because when Damon slept with Rebekah they werent together, and stelena just broke up. And Damon didnt use to sire bond to tell her it wasnt a big deal.

when i say sired i mean she was technically dating and together with damon