Advice! Ex is engaged. Heartbroken. 💔

OK. My ex and I were together in high school broke up for various reasons but the main one was him joining the navy. I was more then willing to move with him wherever he got moved too. He graduated basic I went to Chicago with his mom, sister, and grandmother to see him. It was amazing! I was so excited. I came home and things changed I guess. But a few months later he came back home and never told me. I found out thru his sister, I was heartbroken. Went to his moms house to confront him and see what was going on. He said he wanted to see what else was out there. Which I completely understood.. he was headed to Spain. He said he'd keep in touch because he didn't want to loose me. Said OK and he left. It's been 4 years and I haven't heard from him. He just got engaged the other day to a girl he met in Spain..who he's been with for roughly 8 months. He looks happy and I'm happy for him, I really am. It's just hard to see someone you love go ahead without you. I just couldn't help but feel like that was suppose to be me, in Spain, with him, happy. Is it crazy for me to feel that way? I miss him like crazy after all this time and everything that's gone on in our lives, but of all the people in my life I've said goodbye to he's the one I regret letting go so easily. He's not re-enlisting in the navy, and will be back in the US in roughly 4 weeks. He's getting married in less then 4 weeks. It hurts to think about. My best friend told me to find my happiness in his happiness but it's just so hard! I'm overreacting I know (period hit the other day 😑). Am I crazy?