You really have SO much on your plate! I am not surprised you are having a hard time fitting in volunteering. I can see how something has to give. If you think volunteering will harm your success in your classes, maybe it isn't as important?
GRE tips:
I did well on the Verbal and AW (167, 5) parts, but only okay on quant (153), so I guess you know which advice will be better.
Verbal:
Flashcard apps. Do them daily.. even just 20 minutes when you're waiting for something. Use multiple apps, because they all have their faults, and they also all have different words. The best apps track your progress. Flipping a card does nothing, you need an app where you can log somehow if you knew it or not, otherwise you'll just stare blankly and keep forgetting. Magoosh has free GRE-specific flashcards, and those are great. Do all the lists, twice. If you want to practice words in context, which will really help, use the vocabulary d0t c0m app. I can share my GRE word lists with you, they don't have their own GRE lists so I compiled some which was SUPER time consuming. There are some weird mistakes or awkward phrasing in the questions sometimes due to being made my robots, but this app is sooo helpful. During your day-to-day life, try to use the words. This somehow helps you remember.
Analytical writing:
There are lists on the GRE website of all the possible essays prompts they could give you. Do a handful of these from both types of essays, TIMED. The key with these essays is to get an understanding of what types of things they'll give you, and how to approach each, because otherwise you will flounder. Argument essays are easier, because there will always be either holes in the argument prompt, or at least alternative explanations for their assumptions.
Try to do this for argument essays:
1) restate the premise of the argument made in the prompt in your own words. Follow this with a sentence such as "The conclusions drawn, however, are based on assumptions that are not entirely warranted, which I will outline below". This isn't perfect, but you can tweak this IF you have enough time. Your first task is just to finish the thing.
2) The body of the essay should be 2-3 paragraphs, ideally 3. Each paragraph is about poking a hole or casting doubt about the prompt. Usually, it's an assumption made in the argument, or some other faulty reasoning. You don't have to say the argument is wrong, just that there is no reason to assume based on the given information that the conclusion drawn by the prompt is valid.
3) The last paragraph is the conclusion, which should sort of restate what you've laid out, which is that the argument made was based on false reasoning, and before such a conclusion, or any conclusion, can be made regarding <insert subject here>, more data must be gathered. Something along those lines applies most of the time.
4) IF you have time, you can make this better and more creative or smooth, but having a framework allows you to relax when you get to the end, because gettting to the end is important. Editing comes later. The point is, you learn how to write this clunky 5 paragraph essay in middle school, then in college you learn how to write more creatively and shed that clunky style. Forget college, go back to middle school, because you only have 30 minutes.
Issue essays are harder, because they're asking for your opinion and you could go anywhere with that. My advice is to do this:
1) pick the stance on the issue that you can quickly think of multiple points about. As long as it isn't a deplorable, amoral opinion, it doesn't matter if it is your actual opinion. Your actual opinion may make you too thoughtful and actually slow you down while you reflect on life and truth and what you stand for... you don't need that right now. Pick AN opinion, and get on with it.
2) Restate the prompt in own words, and finish with a brief allusion, or even a strong statement, that refers to your own opinion, which you will now elaborate on.
3) 2-3 paragraphs, each with a point supporting your opinion, or whatever opinion you have adopted.
4) re-describe the issue as stated at the beginning, end with a something that draws your previously stated opinions together into something making the larger point your 2-3 paragraphs were getting at. If you can't do that, then just sort of summarize what you already said in strong tone.
Quant:
Like I said, not my strong suit, but I would have done much worse without proper prep.
Use practice books. Use them TIMED. Use flashcard apps- magoosh is also great for quant. But don't stop there, when you're doing practice problems, if you come across a solution that contains a rule/formula that you don't know/have memorized, make your own paper flashcard. These rules and formulas are just as much a part of quant as actual problem solving.
Overall:
Do as many practice tests as possible, do them timed, do the whole thing at once. Don't avoid thinking about the GRE as it approaches just because it makes you nervous, face it before it happens or it'll jump out at you the day of and throw you off.