If you haven't already, search for 'mono/poly' and read the many, many threads about this topic. You can be mono and he can be poly and both of you together can be happy. But it is hard.
What you are describing is not uncommon for many folks exposed to the idea of polyamory recently. Many people who are more inclined to be mono feel as you do - he will love you less, you are replaceable, you should be all he needs and so on. It is totally ok to feel these things! You need to feel them in order to understand what is going with you. Ask why you are feeling these feelings. Why do you feel replaceable? Why do you feel unlovable? After all, he is still with you - obviously he values you. Otherwise, he would just leave.
You may find that the answers to these questions have nothing to do with him, or with poly, or your relationship at all. We all bring baggage from our prior experiences, from growing up. That baggage is often not all bad either. You may be telling him that you feel unloveable and replaceable because he wants poly when what may be going on is that you are telling yourself a story that you are fundamentally unloveable and replaceable. I use 'telling a story' to illustrate that these things that go through our heads are often what we tell ourselves - but that they are not factually true. You are lovable. You are not replaceable. Your boyfriend's suggestion of poly cannot change that basic fact. Nothing can change it.
However, after examining your feelings and after reflecting, you may be fundamentally incompatible with him if he is serious about wanting poly relationships and you ultimately want a monogamous relationship. If this is how things shake out, it doesn't make you a bad person. There is nothing wrong with wanting monogamy if that is what feels right and best for you. People can make relationships where one person is mono and the other poly work. But it is not easy and it may not be what either one of you want. Let each other go in that case, and be grateful for the lessons each of you learned from the other.
But it is not ok to throw things around him because of what you are feeling. It is not ok to yell at him because of what you are feeling. It is ok to have different styles of fighting. That's really common too. You may not feel that yelling is a big deal, especially if it is something that your family did. But for someone like me, who did not grow up in a family that yelled or even had loud discussions, yelling is incredibly painful and disturbing. I have no idea if your boyfriend is similar to me. But if he is, then that may help explain his desire to be around you less. This is a very bad sign.