Your use of the subjunctive is correct in this sentence. I would only add that you have a parallel sentence structure issue with "paid more attention and effort." You pay attention, but you don't pay effort, and effort is not a verb. I would re-write that section as "paid more attention and put forth more effort."1. If only I had paid more attention and effort in the past, I could have already entered any of those famous universities like my old friends.
"Does college mean the same as university?" That depends on whether you're in the U.S. or elsewhere. In the U.S., college and university are nearly interchangeable (there are some minor differences and points of usage, which I will get to in a minute, but they are both institutions of higher learning/post-secondary education.) In Great Britain and, I think, elsewhere in the English-speaking world, a college is something less than a university (like a trade school), and your substitution of "college" in this instance would be incorrect, because you typically don't need good grades to get into a mere college. I'll let posters from other English-speaking nations elaborate on this, as it is outside my area of expertise.(Incidentally, does college means the same with university?)
Now, as far as the way those terms are used in the U.S.: the difference is really only that a college is smaller. For example, Boston College has 8,500 students, while Boston University has 30,000 students. In this particular case, there are other differences, but those are not related to the college/university distinction.
Also, because universities are larger, they are sub-divided into different fields, which are called "colleges." So, Northeastern University has a College of Health Sciences, a College of Arts and Sciences, a College of Engineering, and so forth. Many of those colleges are sub-divided into schools: the College of Arts and Sciences comprises the School of Journalism, the School of Architecture, and the School of Public Policy and Urban Affairs. At a regular stand-alone college, the fields are also divided into schools.
It gets more complicated, though. When we speak about higher education in the abstract, instead of talking about a particular institution, we always call it "college." (Again, I'm talking about the U.S. here; it's different in other countries.) For example: she is a college student (even though she attends Boston University.) He is the first person in his family to go to college (he attends the University of Southern California, but no one in his family has ever been educated beyond high school before.) They graduated college in the same year (even though they both went to Brighham Young University.) Binge drinking is on the rise among college students (we are talking about all students at four-year post-secondary institutions, colleges and universities combined.) So, in your example, if you are writing for a U.S. audience, it is not only acceptable but preferrable to say "college" instead of "university."
But wait, there's more! Some traces of the distinction between college and university do persist in American English. For example, we have something called a "community college." These schools are a step above trade or vocational schools, but they do not offer 4-year (Bachelor's) degrees. Students either earn 2-year (Associate's) degrees, or they take general education courses and then transfer to a 4-year college or university. Community colleges are not as exclusive as regular colleges and universities; in California, for example, the entrance requirements are that a student:
a) have a high school diploma
b) have a general equivalency diploma, OR
c) be at least 18 years old
That's it--if you flunked out of high school and never even bothered to get a GED, once you turn 18 you're eligible to enroll! I had several friends in high school who used the local community colleges as a stepping-stone to better schools, either because their grades weren't very good (you pretty much have to have straight As to get into any University of California school--they are extremely competitive) or because they didn't have much money. In California, the UC schools work closely with the community colleges, and it is much easier to tranfer from a CC to a UC than it is to get into a UC right out of high school.
Some trade schools also call themselves "colleges," as in Vogue Beauty College, which trains students in cosmetology (that is, cutting hair and giving manicures--not exactly higher education) but no one really refers to those as colleges, even if it's part of their name.
It's all very confusing, I know!
You should say, "I might have crumbled." The verb "to crumble" means exactly what you think it means, but we don't use it that way as an adjective. Other than that, your sentence 2.1 is correct, as is your sentence 2.2. Technically, it's better to say "if I had been you" when talking about a person's past situation, and "if I were you" when talking about a person's present situation, but few people do that--it's too wordy. Mostly we just say, "if I were you..." Sentence 2.3 is incorrect because you wouldn't still be nervous at the time of speaking; the exams are over.2. Given the situation that my friend is talking about his past, when he took two significant exams on the same day, and I reply:
2.1. If I were you, I would have been very nervous about that and might even be crumbled.
2.2. If I had been you, I would have been very nervous about that and might even be crumbled.
2.3. If I were you, I would be very nervous about that and might even be crumbled.
(I wrote this with the idea of expressing that if I were him at that period of time in the past, I might have collapsed at that time since the pressure might be too much for me to manage and the schedule could be fairly tight.)