First off...Great pic, I don't know why you would want to flatten the image by taking away the contrast. Perhaps you just want certain details to como out? I don't know. I think a quick and easy way ( cuz of the # of steps ) is to:
1- Copy that layer
2- Change the blending options to "Screen"
3- Lower the Opacity level till you are satisfied
4- Erase the parts you DON'T Want to change on the copied layer so that the untouched layer shows up. This way you are letting the program do the work and the picture looks less retouched.
There are about 26387628736287 other ways and neither one of em is less convenient or effective than the next but, this is just one more to that bunch
Good Luck!