So I'm in the same sort of situation as you, but I'm the girl who doesn't want to hang out one-on-one. For me, I don't want to encourage him in any way, because I don't love him as anything more than a completely platonic friend, and I know he's still in love with me.
Maybe your ex is doing just that---not wanting to give you the wrong impression about her feelings for you. In that case, I would suggest trying to accept not hanging out one-on-one with her for now (and quite possibly not in the near future), and accept her as a friend. Nothing more. I know this hurts (I've been there from both sides), but in the end it will be the best thing for you both (and it will lessen your chances of you annoying her from here to hell and back=P).
Best of luck:)