From reading your post and responses to advice I can't really understand exactly what insight you are looking to gain...
I understand that you are insecure about your girlfriend's sexual desire (or lack thereof) for you. I think the replies you've been given so far have been in correspondence to you being so obsessed with the fact that she was very sexual in previous relationships but not so much with you.
So, my question is: If you didn't know about her previous sexual history and the both of you were having the same sex life that you're having now, Would it still be a problem??
You see, you haven't made the case that: You are just frustrated that your needs aren't being met or you're deeply concerned about her cheating.
It just seems as if You are torturing yourself about her contrasting sexual history.
Either you trust that she really is tired, stressed and not in the mood for sex (it's not important to her)... while loving her for whatever else she brings to your life OR You have a serious but compassionate conversation about her not being available for you sexually and why not? What can be done to fix this?