A few years ago a client told me she had more trepidation about writing her residency personal statement than taking her USMLE boards. Writers' block - experienced either before starting the essay or before working on a revision - is normal but aggravating.
I recommend two tactics:
1. Make an outline. This advice might sound like something your fourth-grade teacher would tell you, but it works. Start with a five-paragraph template (which can be changed but is a nice place to begin) and jot down each paragraph's thesis (or main point).
2. Give yourself enough time to work on the PS. The season is already progressing, so this is something to note. Writing an excellent essay in a few days is difficult. Sometimes you'll need a few days' break between revisions just to get yourself back on track.
Please contact me for help with your medical school, residency, fellowship or post bacc essays. The sooner the better. Here are updated comments about my work.