What can I do if I'm afraid to move on from the guy I love?

I 'm in love with a guy who has a girlfriend. He has rejected me in many occasions because he is very secretive about his girlfriend and I thought he was single in some points, but he was not. In many cases I've thought he was giving me hints of been interested back to me and I feel that there is mutual attraction. However, I don't feel strong enough to move on because I' m afraid of losing the chance to be with him in case that he breaks with her. I love him so much and I 'm terrified of the possibility that if I find a guy I don't love, and the guy I'm in love with breaks with his girlfriend, I will lose him forever, and despite I know it's wrong, I feel the need to be available for him. H-E-L-P!!!

What can I do if I'm afraid to move on from the guy I love?
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