How can I overcome my emotional problem associated with relationship failure?

Hello doctor,

Okay, maybe I sound crazy admitting this, but there is this ex-best friend I am unable to forget. I find myself looking at his pictures and crying. He was a friend with benefits, so there are those dates I remember. He used to say he loved me, but more than that, he was the only person in my life so important. Now it is about to be five months since we last spoke. I found out that he was lying to me. He had already been in a relationship for a year, but he continued FWB (friends with benefits) with me and lied to me about all this. Now, I feel like I am just stuck, knowing why he did this. I just end up recalling those days which were once special and now feel the worst. Like, what was he doing? And now, he seems amazing with his girlfriend. How do I stop thinking or crying? And why did he do this? He used to show that he cared. I loved him so deeply. Now, I just feel like I should run and stop myself from literally destroying myself for this man. And you know, somehow, I just want it back, maybe. That thing, that happy feeling. I have not been able to trust anyone after this. I have not been able to feel for anyone either. I used to always be online talking to him. Now, I rarely want to talk to anyone.

Kindly help.