Be super, super specific. You should not be able to recycle any part of a "why XX?" essay for another school, because they'll know! (In my opinion, this is especially true of schools in big cities - I'm sure adcoms are sick of reading the "Boston can be my classroom!" essay.) It takes time, and it's difficult, but what you need to do is research each school and find specific things that interest you. Whether it be study abroad opportunities in a certain foreign city that you like, the fact that they use the quarter system instead of semesters, the opportunity to take classes at a sister school... find something that stands out to you. There'll be something, I promise! I also looked through student life websites to find out about clubs and groups that would interest me. For example, in my answer for BU, I mentioned that I enjoy singing and could see myself joining the Treblemakers (an a cappella group at BU).
By the way, don't worry about your writing skills. There are tons of readers here on CC who will be more than willing to proofread and help you out.