some women do have that. i think it's better to consult a gynecologist for this matter. i know it'll be a very uncomfortable situation but there might just be an underlying reason for all this.
it maybe a nerve
, or something to do with your menstrual cycle.
or it could be psychological. maybe your mind wanders off somewhere instead of concentrating with your bf when you guys are doing it. or you're too concerned on reaching it, you end up tired of even thinking about it, hence the lack of interest.
it could be stress-related too. observe yourself for a whole week, list down the things you do in a day. maybe you're just tired because of too many things you have to do.
if everything i suggested still could not pin point what's wrong, then maybe its the boyfriend's fault? he maybe doing it too fast too soon for you? women do take longer than men you know..