I think you should handle this professionally and 1) quit in person, and 2) give them enough time to find someone new. It is completely okay for you to feel like the job is not for you. It IS hard, but would you rather do something you hate for a while or quit before they are completely reliant on you? Let them know, and while you find a job, allow them to replace you. It is a probation period for both you and your employer. The best thing to do, however, is to give them enough time, give your notice ahead of time, and don’t just disappear. That will burn bridges!