First, that is natural, I went through the books 3 times and just now feel like im grasping the concepts. Go through them again. I took notes on my 3rd time and I am beginning to remember the concepts by memory.
Secondly, There are no DS questions, other than the few examples, in the
MGMAT books. What you need to do is go to the back of each
MGMAT book and work on the DS questions asked. Each book has a CH. dedicated to the corresponding OG questions. These are the most important questions to do. One page is for PS and the other for DS. (It took me a while to find where the different question types were in the OG, but once I did, I tabbed them).
Thirdly, know the DS tricks. As soon as you see a DS question on the GMAT your paper should immediately have "AD/BCE" next to it. If you don't understand this, its another good thing to pop into the gmat club browser. Im sure someone has already explained it well.
Lastly, take your time on the problems. There is no use busting through 20 questions that you sorta know how to work, when you can definitely work five from start to finish. You will start noticing a pattern and will then be able to work on timing. You have plenty of time to study. I really believe a 550 is attainable by just knowing the fundamentals. I don't think you hit a wall until about 600 and then you have to hone in more on complex concepts.
Remember to also do some Verbal, even if you feel comfortable with it. I went down from a 38V to a 33V, because I didn't study between my practice exams. If I would have just dedicated some time to verbal I probably would have gotten higher than a 600 on the second go.