How do y’all cope seeing the person you love with someone else ...

We grew up together & started dating in high school. He was my high school sweetheart... but I left to college, he left to the army & didn’t hear from each other again because I didn’t know his address and he didn’t know mine. Not only that, I had gotten a new phone because someone had stolen my old one. His family was always visiting family in a different city so I never got the time to pass by & stupidly I never thought about leaving a note. It’s been about a year and a couple of months. I’ve came back home for a month and I ran into him.. and his new girlfriend. We didn’t talk, I don’t know how long they’ve been together now, but my heart sank to the pit of my stomach. Anytime I see a guy with his characteristics or that military haircut, I get so down. The thought of him with her kills me. I was never able to go back home because I’m also in a military program at my uni & it’s basically basic training over the course of 4 years while getting my degree. I get to go back home for certain things that are pre-approved either by our commanding officers or if our military letter gets approved. What can I do to get over him? How can I not think about the man I wanted to reunite with and spend the rest of my life with together..