Answering an interviewer's question about why you left with "I didn't fit the job" is not necessarily a bad thing. There are very reasonable "technical" reasons why this might be, such as:
Since contract work is not unusual in many healthcare professions, short-term positions aren't necessarily problematic. Different employers will also have different views of what is short-term – and what is problematic short-term. After all, someone could have had a number of short-term positions, each with a different length and different reason for leaving, e.g., travel job, personality conflict, unit merger, relocation, and funding cuts.
One thing that employers are looking for is a pattern. For example, being let go from one job due to a personality conflict is understandable, but if you've been let go from several jobs due to personality conflicts, it's a negative pattern and red flags will go up.
Don't be discouraged. Just because your performance failed in one area doesn't mean you're incompetent in your field. So in stating that you were let go because you didn't fit the job, state why, even if it's a skill you lacked. If what you lacked isn't part of the job you're applying for, it's a non-issue. If it is, you'll just need to explain what you've done to address the problem. The key in such cases is to be honest about your failings and to show a willingness to improve – particularly through actions, rather than words.