No matter how careful I'd act at a job interview, I would never land a job. Why? Do recruiters get turned off when applicants act as carefully as possible? Will I have better chances if I'm not as careful at an interview as I was before?

Being "careful" suggests to me that you are being overly guarded in your responses and demeanor. Interviewers want to get to know whose behind the mask. Many mistakenly think that they have to put on a facade in order to make the interview successful. Any good interviewer wants to get as much accurate information as possible. Your job is to help them achieve that goal. Also, consider getting some interviewing skills training as well. You may find it to be valuable. Good luck.

You have to walk a fine line. You definitely need to be careful--a better way of stating it is you need to pay close attention to your actions to give the best image possible to the employer. But you also need to be relaxed enough and confident enough to let who you really are (all of your good qualities, anway) show through. I would suggest finding someone who would be willing to do a practice interview with you. Ideally someone who has been in a position to interview or hire people before. Ask them for some honest feedback, no matter how painful it might be to hear, and then follow their suggestions. Have them look at your resume, your application, etc. Come dressed like you would in a real interview. I've been in situations where I was the one giving the interview, and some things just jumped out at me that turned me off: People who slouch, or who won't look you in the eye, people who come underdressed, or haven't shaved. Even though their resume looked great, they didn't. Sometimes it's the little things that matter. Good luck.