Salam. The first and only time I had done something haram with a guy was two years ago, this is someone who I had mutually feelings for. I think he lost feelings for me, or got bored. I'm not sure, I never got closure. But I can't help but always miss him and be sad all that time that I gave a part of me to someone who does not even care about me anymore. How can I stop feeling sad about him?
Alaikumassalam wa rahmatullah,
I’m sorry about your loss. I think the first step to getting over a breakup is to acknowledge that the pain is going to be there. It is like a wound that’s going to take time to heal. There is no avoiding the pain and there is no quick cure. This is the stuff that life is made of. Think about why God let it happen and why He is allowing you to suffer now. The Prophet 
suffered terribly in Mecca for 13 years before he went to Medina. God could have instantly given him success if He had wanted. Suffering is what builds our character, and there are two ways of responding to it. We either get bitter and resent God for not giving us a better life, or we accept our situation, knowing that this is where God wants us, and we leave it to Him to take care of our story.
The best cure for getting over a relationship is to get into a new one. Maybe God will make that possible some time. Until then you just have to be patient.