What is the most professional way to quit my internship?

Did I understand correctly that this is an unpaid internship? You should spend some time Googling the laws governing internships in the US. There are very strict regulations about what an unpaid internship can consist of, designed specifically to prevent free labor (essentially, an unpaid internship has to center on educating the intern, not on taking advantage of them.) You could very well have a good employment law case here, and you may wish to consult an employment law attorney. (There are plenty of websites that offer free, basic legal advice.) Regardless, I admire you for taking this commitment so seriously and wanting to end on professional terms, but it really sounds like you are being taken advantage of. Good luck!