Hey there!
I would just like to point out that without even reading your post, I can tell something's really wrong! After reading your post, there's still nothing that changes that opinion
If at any point you're feeling drained, just stop. It's important that you maintain a healthy and positive mental attitude (this includes not overburdening yourself!) for so many different reasons. Not completing questions while drained may be justifiable by your condition at the time, but it's not unreasonable to think to yourself in the heat of the moment that you're a complete failure for not completing 'simple maths problems' as you put it (I've been there and felt this - don't push yourself too hard) - this is so so bad! Also, it will reduce your productivity and efficiency in not only your current session but other practice sessions as well due to things like continued fatigue, lower confidence etc.
A lot of people will have the same problems - it is a learning curve, but you'll find that people who tend to succeed more know their limits better, and when to start and stop. There's no cap or recommended number of questions, quality or quantity-wise that you should be doing - this varies from person to person; instead you should use your limits to gauge how many questions you should do, and frequency and quality of study tends to be the yardstick with which you measure improvement, particularly for more abstract aptitude tests such as the UCAT. It's totally okay to stop if you feel fatigued, because literally no one will judge you, and it's better for you in the long run. It's important also to note that practice =/= results (well not linearly, anyway), and you actually tend to drop off with too much work. This close to the exam date as well, keeping the aforementioned healthy and positive attitude is just that much more important.
Hope this helps