Well, I'm guessing you made a couple of mistakes, here.
As Redhawksrymmer has pointed out, the best thing for you to try would be using Chromanator's grading tools to match the brigtnesses of the clips as best as possible, and feather the masks.
The differences in brightness are probably because you left your camera on automatic white ballance, focus and brightness. To avoid this problem in future, set up your shot as you want it, and lock off all the controls (set them to manual). That way, all the shots will remain the same, and should match up better. Just remember to make sure the shot is focused/white ballanced correctly before you put it to manual.
The second mistake is that you've not really thought about how shadows are going to affect the shot - as you can see, you've stood quite close to the wall, and without very complex g/mattes, there will be discrepancies where the shots join up. Again, this is something you'll have to live with unless you re-shoot, but it's something to bear in mind for next time you're filming.
Hope this helps
Cool costume by the way!