Hello,
I am the same way. I got a math major, with a high GPA, and my quant score on my first one (okay, two) practice tests were much lower than they should have been given my background. I have realized a few things:
(1) This is a test of your ability to take the test. Not entirely your knowledge of maths or ability to write.
(2) It is a test of ENDURANCE. Seldom in college do you have to sit still for 4 hours on end, not allowed to drink water, crack your back, etc, save for a 10 minute break. This was huge for me.
(3) It is a test of STRATEGY.
Advice.
(0) Prioritize which sections are most important. Unfortunately I could see how for psychology they are all relatively equal, though maybe they care less about vocab?
(1) Start by practicing (take a diagnostic test). Buying a Kaplan book unlocks 6 realistic online tests and 2 in-book tests. As you take the first test, write down your weaknesses both conceptually and strategically. After, read over what you got wrong and write down more conceptual and strategic weaknesses.
(2) Shore up your weaknesses.
(3) Repeat.
I studied for about 10 weeks. I would go to the library 2 or 3 times a week, alternating study and practice tests.
The other thing I have to emphasize is that PRACTICE will help you get over the ENDURANCE part of the test. Practice in realistic situations. For me, I drink like a liter of water every hour. Without going into detail, I had strategies for overcoming this.
As far as resources, I mentioned Kaplan. I'm sure the other ones are good too. Just make sure they offer ONLINE SIMULATIONS.
Lastly, I'm not 100% sure studying in groups will be that beneficial. But that might just be my own study habits.
--asymptoticallyAbnormal