When you prepare, your factors are the prep materials then direct score.
When you give an actual test, your factors become the test questions, mental state, the test environment, time management if not done during practice.
You should have mentioned whether you already attempted the test once.
Small things that affect the test day are the extremely cold room and weird marker plastic paper. Its not a big deal but really affected my first attempt ( I am cold sensitive ). I didn't let it affect me much, it was annoying but that's that. But if you are someone who already tenses too quickly, suffer anxiety on the test day, or let small things like one question affect you, you should practice accordingly.
Now coming to time management, if you are struggling with it, I recommend overcorrecting it during the practice, Like do 50 questions in one sitting instead of the standard mock, in case of missing the option to select number of questions (
GMATClub tests and Veritas prep allow you to select the number of questions ) you should practice with a whole mock in one sitting or at the least multiple sections in one sitting. I also suggest you practice on a proper study table chair, without music ( This is basic, I know)
Honestly, exhaustion should become common during practice. You should fix time management to avoid the silly mistakes too, llike not reading the questions properly or jumping to conclusions on the questions asked, happen because of this.
Now coming to your core question, how to measure success, test score is an easy answer, error rate is another. But take into account the error rate per difficulty level, and the time taken to reach it. Some students study 10 hours a day to increase 10 points, after a month those same 10 hours might increase just 2 points. So test score increase per hours dedicated also make a good parameter.
Measure it properly though, Some students think they studied 5 hours when 2 of those were wasted with wandering concentration. Dedicate most of the time to just mistakes, get a 100% correct rate in 500 level questions, then a 100% correct rate on 600 level questions.
Lastly wrt CAT I would still suggest you go through all the gmat videos teaching you basic if you have time. CAT is mostly about core knowledge and speed on basic concepts. GMAT is mostly about tips and tricks and speed of evaluating the method to calculate ( there are often two/three different concepts applicable to an approach ). CAT isn't difficult per se, but the evaluating method makes both the tests different.
Make personal notes too, others' notes are good and all, but sometimes in chasing after difficult concepts, some students forget the earlier learnt concepts tips and tricks.
Good Luck.