1:I look up at her.
我抬头望着她.
2:How mean I had been to her when she was being so nice.
她人那么的好我却这样对待她,我真不应该!
3:All I did was to hold her hands.
我只是握着她的双手.
The "all" in this statement refers to the "action(s) that I did （我所做的行动）",and does NOT refer to "her hands".You can also say,"All I did was to hold both her hands." 但是 “both” 没有必要,因为 "hands" 已经代表双手.