You did indeed face tougher questions in the 2nd quarter as a result of answering all questions in the 1st quarter correctly. While it is normal for accuracy to decrease when the difficulty level of questions increases, the anomaly is the lack of recovery in the later quarters.
For Verbal, the explanation may be that you were running out of time: you spent a lot of time dealing with hard questions in the 2nd quarter and did not have enough time left to conquer less challenging questions that came later. The third quarter of the Verbal section was particularly damaging, as you actually got many easy questions wrong. What is consistent, however, is that you didn't know how to tackle above-Medium difficulty questions, as the difficulty level of the questions you got right hovered right around the Medium mark.
For Math, I'm guessing that you got exhausted or lost your focus somehow. You actually spent less time on questions you got wrong than on those you got right, a phenomenon that tells me you made careless mistakes rather than were unable to tackle hard questions. Or was it the case that you had some questions you simply had no clue about and skipped straight?
Overall, it seems that you have some holes in your understanding of GMAT topics. You don't seem to really have a strong sub-section, so I'm afraid you'll have to get better on all fronts. You may have to start over from scratch, since
the Official Guide itself is not a very helpful strategy guide; it is much more suited for practice. You have several options:
1. In-person courses: These are great for people who think they'd benefit from the structure and peer motivation that come from enrolling in an organized class. The entire study plan is laid out for you, you have regular classes to attend, you have lots of potential study partners available, and you have at least one person (the teacher) who is an expert that you can consult when you need help or advice. A lot of people who are working full-time and don't have the time to put together their own study plan, or the energy to keep themselves motivated and consistently working without an outside influence, tend to get a lot out of these courses. You also don't need to know anything about the GMAT prior to starting one, though even if you have started studying by yourself, going over everything comprehensively can really help you master the topics. You still have to put in plenty of effort yourself, but there are some excellent prep courses out there if you're willing to do a little digging. Unfortunately, these courses are available only in certain large cities. If you don't live in one of these, tough luck.
2. Self-study: Self-studying takes a LOT of motivation and patience. You need to be rigorous about sticking to your schedule, but life happens and that can distract you from following your schedule sometimes; without an organized class, it's easy to lose momentum. It's a balancing act, and the stress of that alone is why it's definitely not for some people. There are tons of internet resources out there that you can use to your benefit, along with some very solid prep books that should serve as your foundation. Some students could have made more progress faster if they had a mentor of some kind to help them out once they started scoring in the upper ranges. It can be hard to diagnose your own mistakes and weaknesses.
3. Online courses: These are a great in-between option for those who don't want to commit to an in-person course, but don't want to be completely on their own either. Online courses will give you a lot of the structure that regular classes do, although you have to motivate yourself to do the work consistently. It's essentially assisted self-study. Some courses give you access to an expert that you can email with specific questions as well.
4. One-on-one tutoring: This is a great option for people who have already spent some time studying for the test or have already taken the test. Perhaps they have worked through some prep books, or completed a course, but they're not at the level that they want to be at. Private tutoring with an expert (make sure they've scored at least a 720, preferably 750+ on an actual test, otherwise they're no expert) will pretty much always guarantee a score increase. This person can help you identify your weaknesses, fill in the holes in your approach, give you lots of useful advanced tips, and help you tailor specific study plans. Tutoring tends to be expensive, rendering it a good option mostly for those that have prior exposure to the GMAT. But if you're pressed for time, or stuck at a certain plateau, this is your best option.
Hope this helps. Good luck!