Well, if your pdoc thinks you should be on 900mg, I think you should listen to what he/she says...
I also started on 600mg, and have not had any success, and so for just under a week now, I have been on 900mgs....I still feel nothing. And just so you know, I also experience extreme anxiety...so what I did was posted a list of "toxic" side effects in my house, and if I happen to start feeling anything unusual I can go take a look, and make sure I'm not in that state....but I have heard of people needing to be on waaay more than 900mgs anyways...
Your pdoc would not get you to go up to 900mgs, if he/she did not think it would benefit you....anxiety is a strange thing, and I know what you are going through, but I really think you need to trust your pdoc, as they are the ones that will help make us stable.
Anyways, I hope this helps you out in some way, and I hope everything goes well for you
-Rsky