You are who you are. You are that because of your programming, your past, your education, upbringing, etc. If this programming didn't include thinking big thoughts, chances are that you never accomplished anything big. Big thoughts create big things!
It's our going beyond our mental and physical comfort zone; it's our going the extra mile; it's our thinking beyond our limits, conjuring up the seemingly impossible that will tap into that Universal Substance just as every other thought we think. So if we do think, why not think big thoughts right away? If we make a habit of thinking big thoughts, they too will become automatic and then "us". And then big things happen just as automatically, too, because it's become totally normal, and we don't have spend much thought about that anymore.
Even if for now you don't accomplish your big dream, at least you've come a lot closer to its fulfillment. And this has lifted you onto a new level, from which it is easier to grab the next rung on the ladder.
Remember, when you have arrived at this part of the MKS, you will be very well versed in physical and mental control, physical and mental relaxation, intense and complex visualisation, PLUS intense and abstract concentration exercises. You're well into the 6th and final month of your study, so thinking big will come easy to you. If it doesn't, do not hesitate to go back and redo previous parts and exercises. This is "big stuff" country by now, but what "big" is, that you and only you will define and keep defining.
Again, make a habit of thinking big, of allowing the impossible to become possible, of living the greatest version of your Self. You deserve it and you are good enough to get it. You just need to make a habit of it. It's really that simple.