How can I better my interviews?

I have had several interviews over the last few months since graduating college in June. I go into interviews upbeat but come out thinking I am just not right for these positions. I have a bachelors in business with a focus on marketing and I'm beginning to think this just isn't for me. I do enjoy certain aspects of marketing like research and ysis, but I am not seeing any entry level positions for this. It is pretty much sales or clerical work. I would prefer anything over sales. Now I don't neccessarily have a confidence issue, but I'm afraid that I am coming off that way in my interviews. I don't have any relevant experience and despite being 23 I look maybe 18 if I'm lucky. If I were a recruiter I would have a very hard time taking me seriously. I just dont feel like I would fit-in in the business world. I feel like I'm still a little kid compared to others around me. I'm also afraid that they may be picking up that this isn't really what I want to be doing and I simply need a job for now despite my attempts to mask this. I talk very maturely and professional but I just don't think I am taken seriously at all. What can I do? I am so unsure about everything. I am thinking that marketing may not be for me, but I will never know if I don't at least give it a real world chance. Am I wrong in uming that some bias exists because of my young appearance? How can I improve my chances in an interview? Thanks!