I have a girl dog who is "fixed" She keeps trying ...

This sounds extremely familiar. I have 2 female chihuahuas that were "fixed" and they also hump just about anything. While it used to bother me, I learned to accept that this is what they enjoy doing so I just let them do it. I even bought them 2 or 3 special toys they can play with, to try to decrease their humping of everything else. I find it a little funny to watch, but I love them both and whatever makes them happy (that isn't of course dangerous to them), I let them do.

I know some other Yedda users had some thoughts about it (you can see the other question/answers here.) Sorry I couldn't be more help!

The "humping" is not necessarily sexual behavior. Your dog probably wants to assert her position in the pack (for her the pack is you and your - and hers - family). The alpha male and female assert their superiority by climbing dogs of lesser status, and your dog is probably a little confused as to her standing.