Control will come with experience. And you're smart to not try to stand until you can relax your grip. It's taken me almost three years to be able to do tight, low-speed turns, and I'm still not REALLY good at it. And on dirt, forget it.
How tall are you? You may need to raise your bars up to be more comfortable when standing. I'm 5'8 and added 4" of rise to my KLX and 5" to my DR650.
The best advice I can give right now is just to relax. I know it's hard to do so. But work towards it. The more you ride, the easier it'll become.
Dangit, I wanna go ride now...
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