Grieving a live person

How do you grieve the end of a relationship with someone very close to you and has been part of your life for a very long time? It is not about finding love elsewhere or that someone better will come along. It is about your friend from adolescence whom you also happen to love suddenly distancing you. I have made so many attempts to have to reach out but he has resisted all of them. He has even refused to meet and I am at a loss. I can't give up hope that things will get better at some point but I don't see how. I told him I loved him but didn't ask for anything in return. Didn't ask for a relationship either. He seemed fine for a while and then suddenly one day he said he found me intrusive and that's it. Things have not been the same since. And yet, I have so many memories with this person and this person has a unique and irreplaceable place in my life. A gap has opened up in my life and I grieve it everyday. I am not someone who gets close to people easily and losing someone I have known for most of my life without any explanation just creates a kind of sorrow that I can't really explain to anyone. It is not that he didn't reciprocate my love or that I can find love elsewhere. It is the person I have known for so many years and have lost makes my heartache unbearable at times. Any precedents here of such things? Of no closure with people still alive? How did you move on?