I think you already knew before you posted, but it's not really that it's too dark, it's that too much of her blends with the background and has lost separation. Perhaps if you have the raw file you could open the whole thing up somewhat and get some of that separation, possibly open her face even a bit more and then burn down the whole bottom and lower left to minimize the distraction of the surface she's laying on.
It's not a bad shot of her by any mean. You've posed her well. She's got a good attitude turning her head toward the light and eyes to the camera, but it just needs some massaging. You could probably crop it considerably and that would help it as well.