Staring at my blank reflection in the window opposite me on a nearly deserted L train, hearing only the rhythmic pattern of wheels on rails, I think about how drastically my aspirations have changed in the past year.
This is a really good sentence, but there is a price to pay. If you mention this scene, you have to use it as ametaphor or somehow keep referencing it, as a theme for the essay... you cannot just say it for no apparent reason. It is good, and I hope you can keep it, but you have to "maintain" it as your theme. :-)
Ah... good! Right here in the intro, you are talking about math and engineering, so at the end of the first paragraph you should refer back to the train thing.. something about math and engineering, as though your experience on the train makes you think in terms of math and engineering.
My time at _______ has been indisputably superb. Don't use fancy dramatic words; instead, add some words so that this sentence will contain more meaning. The first sentence of the paragraph should be a PARAGRAPH TOPIC SENTENCE.
to go into an applied field, which is what I want to do.
such as myself. Don't use myself unless you have used "I" in the same sentence.
This is very good, much more interesting to read than many AO essays. And you obviously are a very methodical thinker.