Recently (although still in high school) I've been at university, more precisely at information science engineering as apprenticeship. I want to become an operating system programmer but I severely lack in math understanding and knowledge: for example I've 10/10 (in Italy we have this graduations) in IT, without any whatsoever problem for understanding and gain more new knowledge on the topic, whereas in math I've 2/10, which is severely insufficient.
I really struggle to understand all the theory (it seem too cryptic for me) and I can't mentally visualize and process math: I literally crash. Still I want to go to university, but since the first 2 years are 70% math, I'm afraid to not pass it and just waste time.
So, are there some good tutorials, books or whatever that illustrate math clearly and could help me out?
Well I'm not a programmer, but I'm pretty sure that you need math to program like the air you breathe.
Especially if you want to become very, very good at it; I mean if you want to understand algorithm running time, do analysis on the complexity of a specified algorithm or trying to find a new one who performs better, you really need to be confident in your math (not to a graduate level, but definitely above trigonometry stuff)
In short, those two years you must study math are the minimum required for a successful career.
I think the single most important thing you can do right now is to start liking math.
Okay, this sounds like a joke, but it really is possible; Now, presumably, you hate it, because you find it cryptic and difficult and weird and etc.
But math is logical, much like a computer program; if you like the latter, you probably like the former too, the only problem being that you have some sort of "repulsion" right now for math. Once you get past that, it shouldn't be too hard to start studying hard and understand everything in time for university.
So, instead of giving you a list of books to study math (your high school books are probably fine, after all) I'll advise you to start enjoying math.
One book I recall that was really fun is "Il mago dei numeri", as I remember I appreciated it when I read it.