Advice on getting an interview for internship

Hello Everyone

I have been trying to get an internship for the past 2 months but have been totally unsuccessful. I am so frustrated becuase I feel like I am a good candidate: I have healthcare experience, I have great references; I have had to apply for these position online; From what I understand HR uses keywords to pull applications-are there keywords? if so what are they or is it because I haven't worked since I went back to school 3 years ago. Why is it so hard to get an internship? Can anyone give me some advice on what I need to do to get to an interview, it will be greatly appreciated