The first thing I notice is that most of your prep materials are practice problems - practice is good, but I'm going to draw a distinction between practice materials and study materials. Study materials teach you how to get better. Practice materials let you practice what you're learning from your study materials.
I'm not familiar with Veritas' book set, so I'm not sure what that includes - maybe those do include everything you need to know about
how to take the test. If not (or if you feel that you've gotten everything out of them that you can), then you'll need to identify some new resources that teach you how to take the test. That's going to come from test prep companies - and, obviously, I have a (biased) opinion about the best books for that. :) So I won't specifically recommend our books, but you might want to do some research to see what other students have liked, or go into a bookstore and flip through some books yourself, and decide from there.
For concentration issues, you can do a few things:
- read a little bit every day. Read on the topics that you find most boring during the test. Science? Econ / business topics? Social science? Whatever it is, start reading for 10-15 minutes every day in "advanced" newspapers or magazines. (Scientific American, Wall Street Journal, The Economist, etc.)
- when you sit down to study, plan out a 2-hour study session. Actually, plan enough work for about 2.5 to 3 hours in case you finish faster than you think. Then, GO for 1 hour, no stopping, no checking email, no breaking for food, etc. Take a 15-minute break and then GO again for a 2nd hour, no stopping.
*Note: don't keep going after the 2nd hour. Take a substantial break. It's more mentally draining to be learning new stuff / making new memories. 2 hours of doing that is like 3.5 hours of taking a test.
- when you do take practice tests, take them under 100% official conditions, including essay, IR, length of breaks, everything.
- when you're reading something that you find boring, think of a friend who DOES like that topic or would find that argument or detail or tidbit interesting. Pretend you're going to tell him/her about it when you're done, so you have to remember the general gist. That'll make you more interested.
Next, read this:
https://www.manhattangmat.com/blog/inde ... lly-tests/Factor this into everything you do when studying and doing problems (and taking tests). If your goal is really to make the best decision in each circumstance (and sometimes the best decision is to get something wrong without blowing a bunch of time!), that will alleviate a lot of the timing issues, and therefore the pressures, that are feeding into your mental fatigue.