I'm engaged but have a crush on someone else.
Why are you engaged if you have a crush on someone else?
I didn't know if she was still unhappy with her ex
If you find yourself crushing on someone else while your engaged then this could be an issue. You may not feel fulfilled with your partner if you are lusting for other people.
I will never tell my fiancee anything.
If you think it might only be an innocent crush and you don't want to have a full relationship with this person I would agree to keep it quiet. It is wrong to string your fiance along if you really feel in love with someone else.