The best advice I would give you is to read as much as you can on this forum. How to cope with jealousy is a main theme--and it IS possible to cope with it and overcome it.
It sounds like you are open to the idea of alternatives to monogamy, but are struggling with the reality of seeing your boyfriend also love other people.
It also sounds like you are young (which isn't a bad thing at all!) and that you will learn more about life and love as you go. Saying that your boyfriend is "the best thing that ever happened to you" is how young-ish people often describe love and relationships. I know that's how I felt when I was 20. But now that I'm 30, I feel like I have a lot more control over love & relationships. Meeting someone great isn't just something that "happens to me." It's within my power to create my own "best things" in my life.
I know you feel like you are mono, but I would actually suggest you date other people to see what works for you. You can keep dating your poly boyfriend...I'm not saying you should break up with him. I'm saying you should actively try to meet other people--mono people, poly people, and everything in between--to gain more perspective on whether you will truly be happy in a poly relationship long-term, or whether the world is full of great guys who can provide a different type of relationship that might suit you better.
I have found that liking more than one man gave me a clearer understanding of how to move past feelings of jealousy. I can like two people and once and it doesn't mean I like one less; so surely my partner can, too.
Also, your parents and your friends' parents might not be the best gauge for the success of monogamy--your sampling pool would be people with kids circa age 20, right? As you continue to experience life, you will meet people of all ages and relationship styles.
Single, straight, female, solo, non-monogamous.