Why not adjust the song to match your vocal range, rather than fight with the song?

If you really want to stay in that key, you could create two vocal tracks. The first track would be you singing in the "high" key. Then create another track, this time singing to the same song, but transposed down to where you can reach the more difficult notes. (You'll want to stay as close to the original key as possible). Pitch shift that track back up to the "high" key.

You can then create the final vocal take by combining the two. Assuming that there are only a few places where you need to use the transposed version, you can probably come up with something pretty seamless.

-- David Cuny
My virtual singer development blog

Vocal control, you say. Never heard of it. Is that some kind of ProTools thing?