Well, here is my 2 cents since this is such a weird problem and I might as well come up with a weird answer.
Have you tried this drill with anyone else? You mentioned how close you two are and how comfortable in the water you are with him. Maybe the issue is you are too comfortable with your boyfriend and just kind of zone out and let him take care of it. Maybe he is being too nice about it and letting you get away with it because he wants to take care of it for you. Maybe if he jerked the Reg out of your mouth when you zoned out once you would come around
not that I am saying he should do that
just saying if it was someone else you might work harder at getting it right