"exuberance" means enthusiasm or showing a lot of energy. if someone holds back a cry of pain, that doesn't really make them seem energetic or excited. from that alone, you can infer that "stoicism" is your answer, but that word relates to a pointed lack of emotion, like when someone doesn't want to lose face or show that they're in pain.
First, it is spelled servile. Second, here is your sentence!
The boy was servile and weak, intolerably vain and ambitious.
May you attach a picture of the essay and the rubric? I may be able to help.