Are you using CH tires ?
This could be the result of a couple of things such as...
Not letting your wheel slip through your hands enough. If you don't let the wheel rotate enough, the tires wont point in the direction of the corner as much as you want. If the wheels aren't properly adjusted to pull you through a tight corner, you will go wide.
If you start the drift to late, you might not even have enough time to let the wheel rotate. If you don't have enough time to let the wheel rotate, you will end up off track. It should feel as you start the drift before the corner so you are ready to feel the rest of the corner through.
I don't know exactly what your doing wrong as I haven't seen you drift, just keep at it and feel it out.
Its very frustrating to learn drifting with a wheel, but very rewarding in the end