Dont worry about the whole ordeal about them being bigger and heavier just stay focused as believe me I am just a bit bigger than Aoki at 150 lbs at 16. Now when you are talking about being clear focused if you mean in a sense focused and more prepared and actually thinking about doing that can be pretty tough to get used too. Personally since its the summer I start training earlier in the morning that I usually would so what I do is right when I wake up I think what I have to do and try my absolute best not to think about anything else.
To clear my mind I just think about what I have to do in class right since I wake up and usually Ill just go for a 1.5 mile run and do some yoga to relax my muscles then Im off to train.
But in some sense I can relate to you like if a friend grabs me as a joke I basically automatically set up my arms to their shoulder or pits to take them down without even thinking about it or noticing it, same way sometimes with me when I train you just don't know what you do and if you train a lot, or do the same drill or move over and over(I assume your a white belt) you will get used to it as your limited so when the time comes you may just pull it off and be surprised. Personally I can't even explain how I feel when I spar or compete with anyone, if its in class or even in a tourny.
Best of luck man!