I'm going to tell my girlfriend that there are people in worse situations
Have you thought of anyone in particular who is worse off?
Yes, there is a guy I know and who is living with cancer
That sounds pretty tough. It's good to remind your girlfriend others have problems too. Some are even worse.
It's not like she's going to die in two weeks
It is good to put our problems in perspective, and to remind others to do the same if they are being too self-centered.  As long as you are not too critical of your girlfriend, I'm sure she will understand that others' needs are also important.