I think it involves the intellect, too. We trust God on the basis of something, i.e. his being in control, infinitely wise and good, etc. So maybe part of it is reminding ourselves who God is. I think we lose touch with this because we deal day-to-day with fallible and fallen human beings who disappoint us, including ourselves. God is personal, so perhaps we unconsciously expect God to be like us in those respects, too.
But I agree that it comes down to an act of will. Maybe we can grow by going beyond just saying the words, “Lord, I trust you” (whether in a general sense or about a specific situation). We could go further and act in a way that shows trust or at least a sincere effort to be more trustful (e.g. letting something go, not dwelling on it, not bringing it up every time we pray, moving on to other things, etc.). What do you think?