I think it would wise to go back to your doctor and explain how the Pregabalin isn't helping much at the dose you are prescribed and that you would like to increase. It can be difficult to be assertive with doctors but you have to do what's right for you. Some doctors don't like to be told they are incorrect or contradicted but if you approach it in a non judgmental, factual manner and explain that you have done a bit of research and discovered that the guidelines say that 600mg Pregabalin is the maximum daily dose.
The BNF, which in the UK is the doctors medicine bible, states that between 300-600mg is the recommended daily dose of Pregabalin for anxiety, it's there in black and white. There are many research papers that have concluded that doses of 400mg-600mg a day are most efficacious. Some people will respond to 300mg a day but the majority will need more.
Good luck x