I'm so jealous

It's eating me up. My best friend, Sarah, she just got engaged. And everyone's so happy for her, and I am too, I really am. But deep down, there's this ugly green monster. I've wanted this for so long. I've been with Mark for five years, and he keeps saying 'someday.' Someday never seems to come.

I hate feeling this way. Sarah deserves all the happiness in the world. She's been through so much. But I can't help but think, why not me? What am I doing wrong? Am I not good enough? It's stupid, I know. I should just be happy for her. But it's like this dark cloud hanging over me, and I don't know how to make it go away. I feel like a terrible friend.