"It's sad when someone you know becomes someone you knew."

It takes two people to form and build a relationship. If someone walks out on you, blaming should not be your first reaction. You may have not given them a reason to stick with you. It could be that you changed, drastically, for the worse and they got tired of trying to talk you out of it, but you never really realized that until they left. If you want them back, you can try your hardest to make them return. But after such evaluation on the matter, there is nothing much you could do if they give up on you, but be strong enough to let that person go. No matter how much you love them. Sure it is sad when someone you know becomes someone you knew, but it is even more sad to see a person continue to hold onto something that the other person let go of a long time ago.