Are you sure that the feelings you have for her are just friendship, with a little lust thrown in? Could you be trying to convince yourself, and her that you aren’t in love with her because you don’t want to risk the friendship?
What happens to make you jealous? For example, is it the type of guy she is with, or is it the things you see them do together? If you are sure you’re not in love with her, what you may be feeling could be more of a paternal type of protective feeling instead of mere jealousy. Maybe you just don’t think these other guys are good enough for her, and that’s what bothers you so much.
I was very close to my brother, and I hated most of his girlfriends when he was dating. They all fell at his feet “because he was so cute.” I was a pain in the butt sister, because I ran off several of his wishy washy girlfriends. He had a harem of space cadets, and I just couldn’t stand it. It was none of my business, but I did it anyway, because I knew he could find a better girl for himself. When he started dating the girl who would later become his wife, he told me she was just “some girl” because he didn’t want me to chase her away like I had done with the others. she made it a point to ask me out to lunch, and I loved her personality. She had brains, and much better character than any of the others. I am glad that he married her because a few years later, he had an accident and became a quadripeligic. If he had married one of the flighty bimbos, they would had headed for the hills. But his wife stuck it out for twenty years until his death, and took very good care of him at home, not a facility.
I’m getting off track a little here. Mainly what I am trying to say is be honest with yourself about what you are truly feeling deep down. If you truly are in love with her, you should either tell her and take the risk, or back off until you think you can better control your feelings. If your feelings turn out to be more of a paternal protective type, you can talk to her and tell her you are concerned for her well-being, and want her to be happy. I think she will appreciate you wanting to look out for her, but only to an certain extent. If she feels you are interfering for either reason, she will back away from you.
So first, do a little more soul searching, and when you discover where these feelings are coming from, discuss them with her. Be honest and tell her that the friendship comes first. if you do that, she can help you with these feelings. You sound like an intelligent and caring man. I think she is lucky to have you in her life. I wish you all the best.