For starters, let me tell you that you aren't alone in dealing with this. Almost everyone is dealing with some sort of grief for someone that isn't dead, whether it be an old friend or some family member. People out there understand you, because they are dealing with something very similar.
Maybe the person you lost adopted a new group of people in their life that made them become something they weren't.
Maybe they turned to drugs and alcohol and are no longer the same person you knew.
Either way, the person you knew isn't here anymore but someone else 'lost' someone the same way you did, so you are not weird or alone.
Next, I want to congratulate you, because having to experience the change in that person was probably heartbreaking and caused you a lot of emotional stress — but you made it through. You put on a brave face during their withdrawal and you put yourself out there time and again, only to be rejected. Finally, you were able to let a huge part of you go, and that takes a special type of person.
It takes someone special to move on after losing someone who didn't pass on, but instead willingly made the decision to leave someone like you behind. You deserve someone who sticks around and understands how much more important you are than whatever bullshit reason they left you.
Just so you know, anyone that can't see how important you are before making the decision to leave doesn't deserve to be in your life. Someone that is able to leave such an amazing person like you behind doesn't deserve to see the greatest moments in your life — and your moments will be better without them.
That doesn't mean you aren't going to hurt missing them because you certainly are. However, hopefully, you realize you can move on and put your energy into loving someone else. You are the better person for understanding the amount of love your relationship was worth, and at some point, you will be happy — without them.
You are a brave, strong soul; you have been through enough to understand true heartbreak. I hope that from here on out in life no one leaves you behind. You deserve so much more than you have gotten in return, so get through this grief and enjoy the happiness life has to offer.
Someone else grieving someone who isn't dead and struggling through the same emotions you are