It's not hard to believe that you were fired for repeated tardiness, but it's hard to believe that it's preventing you from getting another job. Do you live in a very small community and your reputation (accurate or exaggerated) is preceding you? Are you in one of those extremely rare areas of the world that is not experiencing a nursing shortage? Are you older, and although employers aren't supposed to, they're discriminating against you due to age? Were there other reasons in addition to tardiness for your termination?
If you think that you're not finding work as a result of what you write in the "reason for leaving your last job" box on an application form, there are a couple of things to try. First, try applying using a When you do get to the interview stage, or if a recruiter calls and asks why you were fired, explain:
Also, find out from your former manager what s/he plans to say about you if someone contacts her/him for a reference. Finally, it would be a good idea to tell your former manager the same things that you plan to tell the recruiter or the person that interviews you (i.e., the points above).
Good luck with getting back to your career!