What do i do about this girl?

This girl has liked me for 4 years and she has been dating her boyfriend for 2 and a half. We started really talking 8 months ago and well we got really close even though she had a boyfriend. She claimed to me so many times that she is not to crazy about him and really likes me

Today she told me that she said yes to his proposal and I was like what!?! And now I been ignoring her calls and texts.. She has been calling non stop and texting. Have not answered anything. She is writing that she doesnt want to marry him and that she needs and misses me, its hard to lose me. Im not buying it. She told me I still have 3 years before I get married I have time and she said nothing changes between me and her..

My friend told me shes crying all the time and she has never reacted like that with anyone. Im not gonna be dragged a long. I decided to leave her a letter to her friend to give to her. It will say that unless she picks one person she cannot have both and until than she will not hear from me.

Is this right? I feel bad because I really do like her as well.