<human>: Can you provide a brief rundown of the text in within 3 sentences?

I was engaged to someone who I was with for 3 years. I moved to another country for him and we lived together for a year. He completely broke my heart (cheated on me with upwards of 5 people in the time we were together). I know we weren't right for each other; he also cheated on his gf before me several times, and I don't think he's capable of being faithful. However, he had a lot of great qualities as well. Obviously. That's why I said yes to marrying him, and was willing to uproot my life for him. We broke up when I finally decided to move back to the US. This was 8 months ago. 

I did everything you're supposed to do when trying to get over someone. I cut off all contact; blocked our ability to even see each other on fb; I changed my number. I did this for about 6 months. And in that time I started seeing someone else. 

That someone else and I have been dating about 5 months now. Things are fine between us. He's nice and treats me well. But around a month ago I started missing my ex. It really surprised me. It sort of sprang up out of nowhere one day. I tried to push it out of my head for about a week, but couldn't and then I started talking to him again over FB. Nothing big--just a catch up message here and there. "How's this family member?"  "How's that friend?"  "What ever happened to that promotion you were going for?"  
We message maybe once a week, or every two. It's always me that instigates. He's clearly moved on--has a new gf. 

I don't know what's wrong with me. I can't stop thinking about him and missing all the things he had about his personality that I loved. I genuinely think about him multiple times a day. Do I break up with my current SO? i think it's pretty shitty that I've been with my new guy for 5 months and can't get my ex out of my head.
<bot>: I was engaged to my ex who I was with for 3 years. Now I'm 5 months in with new bf. Can't stop thinking about the ex suddenly. What do I do?