Why am I such a weakling?

Emotionally. I'm good- really good- at faking happiness, but it's getting steadily harder. I go through phases of euphoria but I also go through phases of suicidal-level sadness. I can't stand to cry, but it's hard not to sometimes. Tears fall from my face whether I sob or not- they just drop out like rocks. I'm elated at any bit of praise or the slightest compliment, but I'm crushed by any kind of reprimanding or criticism. And yet I'm a very sarcastic, critical person. How come I can dish it out but I can't take it? I hate myself sometimes. Outwardly I just laugh it off- anything and everything off- but when I'm alone everything really takes it's toll. No one understands or cares enough to notice. I don't know what to do about it. I hurt so bad so often but I don't know how to get rid of it, or if I can. I don't think I can. I need someone to pull me out of it, but no one will. So am I just doomed to being surrounded by people but being as alone as if I were in exile?