Charles,
It is inexplicable why women choose guys such as the loser Claire has chosen. If your feelings about her are as strong as I've interpreted them, you owe it to yourself to not give up on her.
The dreaded "friend" status is a death knell--stay away from this at all costs. As much as you want to tell Claire your true feelings for her, causing her to choose between you and her current boyfriend, let's first work on uping your stock. To put it simply, make her want to choose you.
I have no idea how your personal interactions with her go, but if I've read your description right, you're slipping into that safe "friend" zone where you like each other well enough, but there is no tension (at least on her part). You need to work on creating some sparks between the two of you to let her know that you are desireable.
It's a bit of a project, but you can do it. You don't want to spill your feelings for her only to be rejected because she is suddenly overwhelmed. Work on building up the right kind of relationship (more romance, less friend) and then break the news to her and put yourself in a better position.
And, Charles, I'm not saying you have to do this stuff, and by no means am I a relationship guru, but I've been sucked into that friend trap before. There is nothing worse than being friends with a girl you really like only to have her confide in you all her relationship details with another guy. Especially when you want to be that guy.
