Time,..it all takes lots and lots of time. There is no crash-course you can take to feel ok with it.
Your boyfriend and you, sound like you really have your shit together. That is awesome place to begin !
Past that, wether you end up deciding you are mono, or poly, It takes a long time of soul searching. At first, you have to get your brain into a 'neutral state of acceptance'. This means you have to flood your brain with new concepts for a long enough period of time, that the brain has a chance of being objective, against everything you were ever taught, growing up.
Once in that neutral place, is when you will find where your natural tendencies truly lie.
Until then, there will be queasy feelings, and those small moments of panic. There will be the tennis match in your head,...where some days you feel you can accept it, and other days where it feels truly overwhelming.
Maybe those feeling will start to become a foreign concept, and maybe they won`t. Only time will tell that tale.....