Uh, difficult girl, I had a similar one a few weeks ago, I think there are a few different issues with these kind of girls.
At first
self confidence, I think she's having a low self confidence, that's why she's still trying to draw attention from her Ex-BF, and that#s also the reason why she's needing your attention so badly.
The downside is, that as soon as you give attention to a person with low self confidence, that person will loose respect for you, and therefore attraction. Because anybody who respects HER, the girl without value, must be of low value too.
Secondly, LMR, I think that's another issue, and I don't think she does this on purpose, so it's not something she could overcome by simply saying "ok, let's have sex".
Some girls have a huge genetically anchored LMR, I did some research in various female communities a while ago, and there I've seen girls that complained that they really want to have Sex with their BFs, but simply can't due to some urge fundamental fear, phobia, anxiety, whatever. They were really afraid of loosing their Boyfriends, but simply weren't able to get over their own LMR.
I'm not a girl, so I can only guess here, but I think LMR can be really fundamental in a girl, like our male urge to take a look if a hot chick with a nice ass walks by, or the fear of spiders, snakes, etc.
How to bypass such a heavy LMR? I have no idea, but I don't think it's possible through pressure, only if you give her lot's of time, but I never made it that far myself, I don't want to wait ages before she puts out....
My advice would be to dump her, with all that time you're investing in her you could find 5 other girls
