Another saying goes something like "You can only succeed at eternal love once, the rest are just trial runs"... something like that.

Last year my relationship with a girl who was also one of my best friends ended, and I lost a really good friend because she couldn't face me "not as her boyfriend". All I can say is if you're afraid of losing her, can you picture yourself just being friends while she tells you about the dude she's dating? If you can't put up with that, then it's obvious you want to be with her. If you can picture talking to her about her relationships and how much she loves other guys, then maybe being a friend is the best choice.