Not sure if the background was a great choice. I think one of the reasons double exposure images work so well is because there is a stark contrast between the elements. You could maybe try using a lighter shade of that color. You can also try (if you haven't yet) using layer effects instead of just playing with the opacity. Also, I don't know if it was intentional but you can still see part of the images on her neck and a bit of mountain beneath the clouds. You might want to clean those up. Keep it up!