I can relate to this so much! I really got a kick out of the reactions that my professors would have after reading my term papers--it was like they never realized that I had the capacity to develop a coherent argument, much less articulate it with any sort of finesse. In my last semester, for example, my senior seminar professor was on my case nearly the entire semester over my term paper. She required us to schedule regular meetings with her, and, admittedly, I did my fair share of procrastinating, so I was very slow in bringing her anything of any real quality. I could tell that she was expecting me to hand in a substandard and, possibly, late paper. At each meeting, I tried to explain that I had a plan in place, and that once I developed some momentum, the paper would come together. Still yet, I could tell she had her reservations. Finally, when I met with her to discuss my completed first draft, she couldn't quite disguise her surprise when commenting on how well written it was. It turned out, that I received one of the highest grades in the class. I sort of felt like a boss.
I've only tried using writing to convey my thoughts to someone I cared about once, and it didn't exactly work out very well. It was a very sincere and heartfelt letter, but, unfortunately, she was over me. It didn't help that I had, inadvertently, delivered it at a less than ideal time. So I have no useful experience regarding communicating with a loved one through writing.