London made some excellent points, and I want to add this:
I don't think you have to worry that you are really mono. The fact of the matter is when we focus on one thing, it takes more attention, more resources. Think of it like this: what if instead of getting to know your metamour, your focus was a brand new job that is taking attention and energy, and making you less focused on your bf. In that situation, you would accept it as normal and not be worried, but because this is something that is not routine in our society, we tend to give such things as diverted attention more significance, even though it is really the same phenomena as focusing on a demanding job or fascinating hobby.
My advice is to do exactly what you would do to maintain the connection to your bf that you would if the cause of your diverted attention was one of the reasons I mentioned above.
Oh, and once you all get your foundation built, it won't require so much attention to maintain. But right now, you are all in the creating stage.