Confused and need advice

I have been friends with a girl coming on 2 years now and there has always been some sort of spark, however neither of us have ever really made a move. We have been on dates, watched films together and done fun activities. Approximately 4 months ago we got into a relationship with different people. Mine lasting only a couple of months and her still being in one.

Deep down throughout my last relationship I know that this girl (my friend) is the girl for me and she is the one I will be happiest with. However she has a boyfriend; therefore I cannot decide whether to still tell her how I feel. I am not phased by rejection, I just want her to know how I feel regardless. im seeking advice on whether do I still tell her how I feel or not ?