Lack of confidence shines through in any person. I can't say that's the reason you didn't get the job. You may not have been successful for numerous reasons (many outside your direct control).
Some reasons for not getting a job can include: you aren't qualified (you can fix that or you might need to apply for a more junior role); someone else was better (you can't control them but you can work on making yourself better); there was not job or job already promised (it's annoying but sometimes false jobs are advertised so companies can follow a HR process for someone they want to hire)
Being confident does help with your interview and connecting with the interview panel. In entry level/graduate positions, your soft skills (communication) can be the key differentiator in being selected.
I hope the advise in this thread helps you a bit
Regards
Colleen