In my personal opinion I would bet money that the relationship you're describing wouldn't have a future. I've been in a very similiar situation (Albeit with someone that sounds like more of a bitch than the girl you're describing) and all I can say is that women can be completely blind to how you're feeling about them. Although you say that you've tried to hide your feelings from her and you've given the reasons why you did this, I'm pretty sure that as a male I could have read the signs and if she'd wanted to she could have as well. Sadly I would reckon that whilst she seems to be slagging her boyfriend off to you to some extent, if you were to get full on and join in she would probably start defending him. Also, women are also much better at maintaining friends (even VERY good friends) without ever openly considering them sexually. If she was to find out how you feel she would either over-react completely, potentially never speaking to you again, or she could ignore it which would be a subtle hint that she didn't want to hear it.

The chances that she would dump her current bf and run into the arms of prince charming (you) are very low. If it was going to happen it would have happened by now. Just stay her friend and make sure that the quality of your relationship hasn't been masked by the fact that you want to impregnate her.

Of course, I could be wrong. Sorry for my advice but I learnt it the hard way.