I knew within weeks of starting that I'd made a mistake

• If you stay longer, it might get better - or maybe not! You could talk to someone in the Human Resources department. 
• If there's a poor match between you and the company, the company might prefer to not spend the money to train you if you're just going to leave. 
• If you find yourself in this situation, contact the Careers Service at your University and talk it over with one of the Careers Advisers. If problems at work are causing you to consider leaving, it might be helpful to talk to someone outside the organisation about it.