It probably isn't fair but very little is. Also fair is relative, I believe. I don't know why they told you to write them, but I don't find it surprising they did not write back. Any number of therapists write about the end of therapy being a grieving process for the client even when it goes well by their definition (which I doubt is the same definition a client would use) - I think your grief may be usual and like grief in other areas, it will take time to get through.