This discussion may contain spoilers!
I noticed a few people in a few separate threads alarmed about how much Clementine's personality has changed in season 2. You have to really weigh up what she has gone through up to this point, and the lack of emotional support she has received.
GUILT: Lee, Omid, Kenny, Ben, all die indirectly due to Clementine falling for the strangers lies, or in Omid's case because Clem made a survival mistake.. It is highly likely that Clementine finds herself personally responsible for this.
GRIEF: Ed and Diana (her parents) Lee, Ben, Kenny, Lilly, Ben, Katjaa, Duck, (Carley or Doug).
These are all people she got to know well, three of which had a very personal parental style relationship, and two of which (Ben and Duck) were good friends. Half of these deaths she was a witness to, which is a lot of grief for a single person to bear, especially a chid.
LACK OF EMOTIONAL SUPPORT: Clementine survives with Christa for TWO YEARS before they get separated. After Omid's death, Christa becomes closed off and emotionally cold. That makes two years of trying to cope with that massive amounts of grief and guilt BY HERSELF, compounded to cause the change in her personality within that time. I don't think I have to point out how much Clementine's life would have sucked while she was with Christa. As you might see, there is a really good reason why Clementine changed as much as she did in season 2.
SURVIVALISM: Clementine has been living a life of pragmatic survival for two years now, which requires you to maintain a vigilant mindset. Twice in season 2 we actually do see Clementine's childlike nature shine through, once with the bottle, and once with the Sam the dog, and in true walking dead style, these moments where she has let her guard down have ended tragically. It's no small wonder why she is no longer the hopeful little girl she was in season 2. The world no longer allows for such luxuries of the mind.