You overwhelmed the sun with the flash, which is potentially fine if you do it right, but I suspect you actually went too far with it this time. Lighting outdoors is really tricky and I am by NO means the master, but what I can tell you is this... shadows add drama. You do want SOME shadows, but how intense they are and where they are has a significant effect on how deeply we feel the impact of the image. In this case, the lighting is pretty flat/consistent across the front of your face, the tree, etc.
I would try turning the flash down and moving it more to the side (if you can). I suspect your flash is on-camera, pointed right at you. You should also diffuse it.
I would also try just shooting using natural light. As long as you're not totally squinty eyed with skin glowing back at the camera, sometimes this gives you a wonderful result. Just depends on what you want.
BTW, I thnk you could also crop quite a bit out of this image... too much tree and dead space over your head for my liking for a shot of this type.