Is it normal to feel this way in this situation?

So this story is fairly complicated. For obvious reasons I'll make up names to call my best friend and so on. So I dated this girl, Jill, for about 2 years. We broke up mainly because we were just growing apart and getting tired of each other. There were no hard feelings and we became best friends. It's been a few years since we broke up and this past summer, our feelings for each other grew. However there was another guy who she likes, Bill, who she previously dated before.

Bill is better looking and more outgoing than me, but from what I've heard (from Jill and her friends) I was clearly a better boyfriend. Throughout the summer, Bill and Jill were kind of going out but not really. Once summer ended, I became busy with work and school so I didn't hang with her much.

One night on the phone, after a long talk about her being jealous over some girl I knew, it was revealed that she liked me more than just a friend. Me being an idiot never saw that all summer. I told her that I also loved her. Everything ended happily right? No. This conversation happened a few weeks after Bill and Jill started to get serious. So they broke up? Nope. I told her to do what she had to do with Bill whether it took one week or a few months. I felt that it wasn't fair to Bill to just do that.

It's been a couple of months since then and we're still best friends. My problem is that I can't stand them being together. It's not that I don't like Bill or I'm jealous, it's just that I'm afraid of losing her. She could easily stay with him for a long period at which point I wouldn't be able to take it. I could wait a few months but there's a point where enough is enough. She said she wanted to go out with me a couple months ago, what happened? I'm not trying to come off as mad or anything, just sad and worried. I know we love each other and I would do anything for her. I don't know sometimes I feel that she thinks that she can just run the course with Bill and once that's done, I'll be waiting for her. I feel kind of used, but is that just a flawed way of thinking or my instincts?

P.S. I feel awkward asking for peoples' opinions online but whatever.