Is it okay to tell a girl how you feel when she already has a boyfriend?

There's this girl at my school, we both used to hang out together, talk, and text, a lot and we were pretty close, we even held hands, it was like she was my girlfriend but it wasn't official.. We both knew that we liked each other, but 2 weeks ago we stopped talking because she said something was going on.. so I wait and during this time I'm really depressed and sad. Recently, she put a note in my locker saying that shes in a relationship now.. now I'm ******* sad but I can't let her know because I don't want her to worry..

We still talk and text a little, but its like a ******* wall is blocking everything, like I cant tell her anything now because shes in a relationship. even though her boyfriend is like half way across the country in another state, it still feels like I can't tell her anything.. Apparently her boyfriend and her used to date, but he cheated on her.. and she still goes back to him..

I still like her but I can't tell anyone because I don't wanna break up a relationship shes in... I can't tell anyone, but if I could, she's the only one I would tell, and I would tell her how I feel about her...

I just don't know if this is the right thing to do.. I'm willing to take the risk of telling her, but what if I get rejected? we wont be friends anymore right after that? I don't know anymore.. please help..