So... Why are you failing the tests?
I mean, something is going wrong somewhere, it's either your instructors or you're just not fixing the problems you have. 120 is a hell of a lot of hours to sink into driving and not have picked it up by now. Especially since like 80% of the test is just exaggerating your observations (had 4 vehicle tests (Car, PPT, PPT refresher, Scissor lift), it's all the same, just drive like a sane person and make your observations clear and you'll pass easily enough).
One thing stands out to me though, your point about multitasking. The fact of the matter is that humans, in general, are terrible at multitasking. That right there may well be the key to your issue. You're trying to focus on too many things at once. You need to learn to disconnect your conscious mind from certain actions and let your unconscious mind take over while you use your conscious mind to do something else. That is to say, while driving, don't keep telling yourself you need to do this that and the other. Just observe and let your unconscious mind do most of the driving.
It sounds odd but let me throw a few scenarios around.
Your tongue, it just sits there in your mouth, you're not particularly aware of it, you just know it's there, almost as if it's floating in your mouth. But wait, you're now consciously thinking about your tongue because of this, now it feels weird, almost like it isn't sat in your mouth right.
When you're driving, you notice you're a bit too far over to the right so you adjust. You then continue to focus on your road positioning, as a result, you keep correcting for slight movements and you end up slowly weaving from one side of the road to the other. When before, you were keeping to a relatively straight line without much thought.
When I back into my garage, I notice that if I focus on needing to be a bit more over to whatever side, I end up parked at an angle. When I just look to make sure I'm not going to hit anything and let my unconscious mind take over the actual manoeuvring, it's usually bang on, maybe ever so slightly skewed.
It's the same idea here, the more tasks you focus on, the less attention you can divert to other tasks, as a result, it all goes topsy-turvy. Just look around to make sure it's safe to do whatever it is you need to do and go. Don't think about the finer details of each action like having to steer a bit more to the left. Let your unconscious mind deal with the finer details, use your conscious mind on the more important parts of driving.