To the poster who's never heard the word 'ironic' used to mean 'coincidence', have you not heard the Alanis Morisette song of the same name? The song is pretty ironic, as all the ironies she mentions are not irony at all. Isn't it ironic, doncha think?
As far as the clauses go, yes, if joining an independant clause to a follower using however, it should be "; however,". As far as the incorrect word usage goes though, and this probably applies to the discussion of irony above, a certain amount of language evolution is inevitable in society. Always remember, dictionaries exist to follow the language, not the other way around. If society decides that the word "hat" should refer to underwear, and it enters common usage as such, it's going to be changed in the dictionary to reflect that. However, in common writing, it's always a good idea to follow the current convention, not the current fad.
As far as commas go, they are the most overused, punctuation by students, who are in college, like, ever. Apparently these people don't read, what they're writing in their, head, and thus don't realize, that, NO ONE would pause in speech, at those locations in the, sentence. Proofing freshman comp papers, made me lose faith in the future of, humanity,.
My biggest concern for the OP is not the grammar issues, but the content. Are you really trying to describe that some revelation occurred in which you realized that medicine would be more fulfilling than waiting tables. It makes sense to describe such a transition from a career like engineering, law, business, teaching, or whatever, but I think it's pretty much assume that waiting tables is a transition job for most people. An explanation for leaving a job like that isn't usually necessary