I am concerned that I am not smart enough

No one is smart enough.

But if you mean, specifically, smart enough to

aspire for rationality

then I think the question is kinda backwards. "Am I too stupid to try to improve my thinking?" -- it's like "am I too sick to try to improve my health?" or "am I too weak to try to improve my strength?" or "am I too poor to try to get more money?".

Now, no doubt all those things are possible. If you really can't reason at all, maybe you'd be wasting your time trying to reason better. And there are such things as hospices, and maybe some people are so far in debt that nothing they do will get them out of poverty.

But those are unusual situations, and someone who is headed for a good result in a challenging subject at a good university is absolutely not in that sort of situation, and if the stuff on Less Wrong is too hard for you to understand the fault is probably in the material, not in you.

I find it difficult to multiply two 2-digit numbers in my head

A fine example of the kind of "easy" task human brains (even good ones) are shockingly bad at. I just attempted a randomly-chosen 2-digit multiplication in my head. I got the wrong answer. Am I just not very intelligent? Well, I represented the UK at two International Mathematical Olympiads, have a PhD in mathematics from the University of Cambridge, and have been gainfully employed as a mathematician in academia and industry for most of my career. So far as I can tell from online testing, my IQ is distinctly higher than the (already quite impressive) Less Wrong average. It is OK not to be very good at mental arithmetic.

(Having said which: If there were something important riding on it, I'd be more careful and I'm pretty sure I could do it reliably. I did a few more to check this and it looks like it's true. So I may well in fact be better at multiplying 2-digit numbers than you are. But the point is: this is not something you should expect to be easy, even if it seems like it should be. And the other point is: Even if you are, in some possibly-useful sense, less intelligent than you would like to be, that is not reason not to aspire to rationality. And the other other point is: It's clear that your intelligence is, at the very least, perfectly respectable.)