Some said the long hours and paperwork caused them to have panic attacks.
But most are satisfied with their training, and 97 per cent felt very or fairly confident that it prepared them for the job.
These were some of the findings revealed in a large-scale study of students in England on initial teacher-training courses.
The researchers from Nottingham and Leeds universities and pollsters Ipsos MORI said their findings "reveal a mismatch between what trainees expected teaching to be and the reality".
"Some interviewees appeared to find themselves on a hamster's wheel, which is how they perceived teaching - which in some cases led to despair and even panic attacks," the report said.
The researchers spoke on the telephone to 4,790 student teachers in 2003, then interviewed 2,958 of them who agreed to be contacted again in 2004.
About 7 per cent had pulled out of their courses, most commonly because they "could not manage the workload".
"I didn't realise there was so much pre-planning and paperwork," said a trainee primary teacher in her early 40s.
Some described panic attacks they suffered as a result of the paperwork.
"I sat in a cafe for an hour-and-a-half and I couldn't even plan one Year 7 lesson," said one trainee. "When it gets to that stage, the only way to go is to say, 'I am not cut out for this.'"
In some cases, poor pupil behaviour tempted trainees to quit. "I thought it was going to be difficult, but I didn't think it was going to be as difficult as this," said a trainee in her late 20s on a PGCE primary course. "It is very hard to motivate the children without them crawling up the walls."
Younger students tended to be more positive about their courses and were less likely to drop out than older trainees. Male trainees were twice as likely to quit primary training as women.
The researchers said negative phrases such as "floods of tears", "feeling inadequate" and "plummeting confidence" were used by many trainees in the study.
But they found that the students used less emotional language than when they first started training, and often talked positively about working with pupils and colleagues.
Some trainee teachers said working in the classroom had been more pleasant than they had expected.
"I thought I was going to get a class of horrible kids," said one trainee secondary ICT teacher. "But you can find you get a class of kids that make you smile because some of them are so witty and amusing and funny and nice - and only one or two of them are challenging."
Becoming a teacher: student teachers' experiences of initial teacher training in England is available at www.dfes.gov.uk