humility and sincerity are valued traits in medicine. if it were me, i (hope i) would go back to the person i upset, tell them how, in retrospect, i wished i had handled things, apologize, and ask if s/he would have me in their class. if you are being sincere, the professor will know and you will have demonstrated your maturity and ability for self-reflection. taking all your science classes as a post-bac will take a good 2 years and an enormous sum of money. confront your mistakes now- there are important lessons here for any premed.