Is it normal to constantly get depressed?

about two years ago, i met this girl and fell deep into her. it didn't go well from the beginning but for some reason I just couldn't stop thinking about her for almost 1.5 years. the thing is when I see a girl looks like her, even I'm in different state, I think of her and hope it's actually not her since I went through really tough time liking her. I was all depressed while I liked her because there was no progression between me and her. no it want just stalking or whatever. I went on dates with her and stuff. well the thing is the depression I had is killing me now. I constantly get depressed out of no where. even after a whole year! when I'm with my friends, I joke around but then when I keep myself quite for even a minute, I get depressed. I hate this.. even listening to "the fray" makes me sad... and I suck at faking expression that I just can't hide it from my friends. is this ever normal to anyone? what can I do?