It's kinda crazy but I get you cause I was in the same position , My friend and I were really close and I was falling for him like crazy but he had a gf. First of all, remember he's you best friend so you have to support him, respect the fact that he's in a relationship. Pushing boundaries, forcing yourself on him or trying to ruin his relationship will crack your friendship.
Try to find someone for yourself, you need to focus on a positive friendship and put his happiness before yours, that's the only way to prove you really love him.
