Yeouch! Sorry, that just struck home a little. :yim_rolling_on_the_ But hey, if I were to steal the covers it would be while I was asleep, not on purpose! Seriously, as an ISTJ I constantly have to be thinking about being sensitive to interpersonal stuff and not be like a tank. I don't notice a lot of the finer details without trying really hard. I will do all sorts of things and go to pretty amazing lengths for people if they ask, but I am not likely to notice quickly that someone needs their glass refilled.She's the type, for instance, who might donate a kidney to a dying acquaintance, but steal the bed covers at night.
I really don't think it sounds like she is manipulating you in any way. As for the not sharing insecurities thing, for one thing she probably doesn't think about that sort of thing nearly as much as you do and for another SJs often have a hard time expressing deep emotions if we even recognize their presence.
I know I would love getting long letters from someone, so I bet she really likes that and saves them all.