I agree with above post. However, if above doesn't work it could be that she is just too selfish or too self absorbed as a person (Or when in that particular mind set) and you will need to ask yourself if this is something you are prepared to continuosly tolerate in the future.
As Junsui correctly pointed out, her happiness should first start with her. You can comfort her and empathize, but at the end of the day you can't absorb her problems and if she thinks that having a boyfriend means that her mood will always be transformed to "happy" just because she is with a boyfriend, she should probably do some self instrospection to find out excatly what causes her to think this way. There could be additional psychological problems that both of you may not be aware off...
And, hey Junsui is back, Yay!!!
Don't cry, don't regret and don't blame
Weak find the whip, willing find freedom
Towards the sun, carry your name
In warm hands you are given
Ask the wind for the way
Uncertainty's gone, your path will unravel
Accept all as it is and do not blame
God or the Devil
~Born to Live - Mavrik~