It's not easy to hold back such strong feelings. It might take a long time for you to redirect your hope. If you accept it is going to be a friendship, you should keep boundaries in place to respect his other relationship. I was in a situation like that where I opened up to a girl I knew for 4 years after she just got engaged. I thought it was my last chance. She got conflicted, but after she went home and called the guy, she called me to tell me we could just be friends. In my case, the friendship in reality ended that day and it felt I made a choice to risk the friendship by telling her how I felt that day...I am not sure.

Those are tough emotions to sort out.