It never helped me. I told him in counseling, with the counselor there. Several times, several counselors, over several years, actually. That helps ME see how long and how hard I tried. Writing him a letter that I didn't give or in my journal helped ME see how much longer I stayed when the situtaion never changed - except to deteriorate.
Telling my AH how I felt only served to alienate him and increase my frustration with my marriage and with him. That wasn't really helpful to anyone.
Sorry.