난 나쁜 학생에요. ㅜㅜ | I'm a bad student. TT
난 나쁜 학생이에요. and the meaning of sentence is right^^hehe

뇌를 집중하고 없어요.
My brain can't focus.
뇌를 집중할 수 없어요.
can=할 수 있다/can't= 할 수 없다. and the other various forms, you can think.
will be able to= 할 수 있을 것이다.
could= 할 수 있었다.

너무 좌절감을 주고 있어요.
I'm really frustrated.
your korean sentence is right. but the meaning of your korean sentence is " it is really/too frustrating"
so, exact answer is "나는 너무 좌절하고 있어요."


난 어쩌면 모든 밤을 자고 없을 거에요.
Maybe I won't sleep all night.
["Maybe I'll stay up all night"? How could I say that?]
난 어쩌면 오늘 밤에 푹 잘 수 없을 꺼에요.

대학교는 상급생을 정말 어려운에요.
College is very hard for seniors.
대학교는 상급생들에게 매우 어려워요.

나의 저 신념에요.
That's my belief.
저것은 나의 신념이에요.
---

Possibly (probably) has many errors. I am tired, but can't sleep. :/ Oh well.


i wanna tell you more in detail, but now really i have to go...ㅜㅜ
bye.

just now come in italki to check whether you are here..bye