Falling in love with my best friend who is in a relationship. What do I do?

I really need some advice on what to do. For me this is worse than torture, having feelings for my best friend without being able to express them. :upset:
We both study at the same university in California where we have been in the same class for the past 4 years. I have always had feelings for this girl, ever since we first met. Something tells me that she also has, or alteast had feelings for me as well until she met her boyfriend. When we were drunk she actually told me she had a crush on me when we first knew each-other. Now when I think back on it, it really becomes obvious that she liked me and that she did put a lot effort in showing it. Somewhere deep inside I knew she had feelings for me and I for her, yet I ignored them and let the time tick away. The reason for it is because I was afraid of commitment. We are really connected and close to each other,and yet, we are so different. Me being a somewhat modern catholic and she a free spirit and a little naive who enjoys drinking and sleeping with as many guys as possible. Even though I had strong feelings for her, I thought to myself that she wasn't the right girl for me. I starred blinded at her faults and not how fantastic she is. So, we stayed friends and she met her dream boyfriend who she has been in a deep relation with for 2 years by now. They are a perfect match and most important, he makes her happy. They have already started to plan their life together, planning on moving across the states and build a house in Florida.
By the years we have developed a really close friendship. We can almost talk about anything to each other and we have so much fun together. She is the only person that I can be myself with, who really knows and respects me. There are no words that can express how grateful I am having such a friend like her. I wouldn't want to lose having her in my life. The problem is that I have grown feelings for this girl. I think of her all the time, everything I see reminds me of her and when I'm with her, it feels as if my heart is melting at a thousand degrees. When we are together hours feel like minutes. I would walk around the whole world for this girl, with no food or water, but purely with her in my mind. My feelings for her are torturing me. There are so many things I would do for this girl but deep inside I know that I am not right for her, and that is what hurts me the most. I can see that this guy she has met is right for her, I feel it when she speaks about him and looks at him. All I want is to see her happy, seeing her smiling makes all the suffering worth it. I have tried to ignore my feelings but I can't, all I want is to be her best friend. We only have a few mounths left in school until she moves away and I have planned to write a letter with my true feelings, reading it up for her the last day in school. In the letter I would tell her how I feel about and how happy I am that she has met such a great boyfriend. It wouldn't be a letter trying to win her over, but rather being honest and tell her what she deserves to know. For me it would be like dropping a huge load, a burden that I've been carrying for 4 years. But I worry that it will effect our relationship.

Please, what should I do? I really dont want to loose her as a friend, but in the same time I can't keep carrying this load.
I really appreciate any help, this is the first time I have ever opened up about this.

Distance yourself, and move on. Tip: stop being friends with girls, you date them instead.