Yes, Nicolas,
That's the direction. As you yourself can see, one quickly loses the power as we get brighter.
This is a complex issue which can only be solved by carefully working in the form of the tree in the left and its trunk but not losing the magic of the sunset in the original picture. The tree on the right can be kept dark, very dark in fact. However, it's the tree in the left which is the challenge to harmonize its whole form with the rest of the picture.
If you have the RAW image, then one can re process as two layers as I suggested above.
Taking the picture 20 minutes or so earlier, would even be better perhaps. This is where, BTW, the extra 1.5 stops of the Leica M8, the Leica R8 R9 with the DMR digital back, the Canon 1DIII or else a MF back with 14-16 BIT pixels, would allow one to optimize this difficult scene.
Jessica, taking a picture of black against white as in a silouhette is , of course, not difficult at all. Taking a rich tonal picture of the brides dress with shades of white against the grooms dark suit with many intermediate dark tones is much harder. This picture of a sunset, so deceptively simple, is even more difficult IMHO to render to the extent it deserves.
Keep at it! You are doing a great job!
Asher