Personally, I think she needs some good professional help. I would bet she has low self esteem, but does not know how to overcome it. It could be bipolar disorder, which means her lows are abnormally low, and her highs are abnormally high. OK, that being said, having been there, done that, and had some wonderful people here lead me to my answers, what would happen if you got tough with her. If she becomes a star in her career field, she will find something else to be unhappy about.
Some people have the gift of contentment, in almost any circumstances.
Therefore, after getting married, that type of person generally does not get better. With intensive professional work, she may be able to be trained to cope better, but it will always be a struggle. When things aren't going perfectly elsewhere, she will bring the problems home and your calm, supportive coping will be taken full advantage of, without her even realizing that she is doing so, or that this relationship is supposed to inculde some kind of equitable balance of giving. If you try to logically work it through with her, she will be angry, find one instance where you were depressed or out of sorts, and equate the two of you. I'm afraid that you have to decide if you love her enough to deal with this basic personality for the rest of time.