My best friend is becoming close with someone else and I am sick with jealousy

I often feel like a worthless person. This new guy sounds really cool and like he has a lot to offer my best friend... As a friend. I know it is selfish for me not to just be happy that my best friend is meeting other people, but if I'm really as worthless and unloveable as I think I am, he'll replace me in no time, and that idea is just devastating to me. My best friend is my everything and I don't want to be shelved, but as my self-esteem is so low, I don't understand why anyone wouldn't shelf me. I feel like my shelving is inevitable and I want to cry.