It's your personal statement. You can say anything that you want someone to hear/read.
That said, there're sufficient differences between FM and IM that you don't have to discuss why you are not interested in FM to apply to IM. You can save that for the interview when someone asks you.
Hopefully, you've put more thought into picking your career. Most people who are miserable in IM are the ones who've "ruled out everything else" and left with Specialty X. It's not a multiple choice exam... There are plenty of threads regarding this topic in all specialties on SDN. Good luck.