The other comments are right when they say you need to work hard - the college is far harder work and far more stressful than university. However working hard will only get you so far.
People also fail due to lack of ability, or ability to pass exams as many trainees find exams far more stressful than live traffic.
Others fail because they give up and get training fatigue, either from the grind of constant training for years or after they've had their confidence beaten out of them by instructors.
You need a thick skin and to keep your head down. The majority of instructors on most sectors and watches do want most trainees to pass but you only need a couple of instructors take a dislike to you or decide you won't pass, and they can make life very difficult for you.
As G_K said you'll have to put up with cross-instruction - where instructor A lays into you for doing what instructor B told you to do the session before (and vice versa). Eventually you'll learn what style of controlling each instructor requires and they'll stop swearing at you.
Remember the job's far easier and less stressful once you've qualified and don't have someone breathing down your neck waiting to scream abuse at you for doing something differently to how they'd do it.
Don't complain (about cross-instruction, incomplete or dangerous handovers, bullying or sexism/harassment), try to fit in, work hard, don't give up and with luck you'll be ok.