Why am I so emotional lately?

I feel really sad sometimes just doesn't know why, I can't stop thinking about how sucks i am when i look in the mirror, and my thoughts are even negative. I always feel like crying a lot . I don't know whether I have hormones problems a not. But sometimes I just feel like I hate myself and I feel worthless and unimportant sometimes and I even feel like smashing the mirror (but I won't, I don't have to do it i know). I know that i'm really Skinny but i have already tried so hard to eat as much as i can but just couldn't get myself to gain more weight...i am now only 48kg which is very very light i knew that and that's what i am always stress about all the time..what should i do??