i want some help here please

hi ppl, im deepak, 23 years old,the situation im dealing with has to be explained, so please take time to read,.
so college starts off, i get to know this girl, she's beautiful ,smart and witty, sneha, we became good friends, often looked out for each other and cared, by this time there is another guy in our class, Rishi, who also happened to get close with her, it was more of him who made the move, always chatted with her, and always used to tease her and stuff, it made me jealous indeed, but if i had shown that jealousy ,she might have thought im possessive of her, so i shut up and didn't appreciate their friendship ,especially when she used to come and talk to me about him, i just changed the topic. Recently i came to know that rishi had asked her in consent for a relationship, and she said yes, i heard all this through another friend of mine, (because she knew i was kinda close with sneha), and this news broke my heart, i didn't know why sneha didn't tell me about this, still i was waiting for her to break this to me, but she never did, so eventually i had to ask her about this stuff, and so i did and she said to me sorry for not telling me and that yes ,all i heard was true, she likes him and he too does. Now here i opened up about my feelings about her cause i didn't want to feel regret about not telling her.
we still talk with each other, very formally, not like before, i know she is in relationship with this other guy, and that idea hurts me very much whenever i see her or talk with her, i think i must stop talking to her, one - because there is no way me and sneha are ever going to be in a relationship, she even said me that rishi is the best guy she has ever met and that she is very happy, and two- im not able to focus on my work, she was one of my good friends and it is very hard for me to picturize her with anyone, and she telling me that she's gonna hook me up with someone, these things hurt me very much.
(i know as a good friend i must be happy for her that she's in a good relationship, but sorry, how much ever i try, i can't, we both have been real close , best way to help both of us is me walking away out of her life)
And the worst part is, she's asking me to stay with her, as a friend or anyone, and that she can't loose me and that im a very special guy in her life etc.
ive got no idea, what to do,.
shouldn't rishi be the special guy in her life? im already hurt, one coz she never told me about that guy coming and asking her for a consent for a relationship, in spite of me being her bestie, ? and me hearing it from another friend of mine,. and two- that ive lost her to another guy,. and the fact that she perceives rishi as the best guy she has ever met.