Tricky indeed, and I myself haven't been through such a situation as I'm an unpracticed polyamorist. However, from what little I know, I would say for you to remind yourself that polyamory isn't the criminal here. Really there isn't one. You live a non-monogomous lifestyle because that better fits your views of life, and it didn't fit his, and it created too much of a problem for the two of you to handle being together with. It's just one of those, "It is what it is" situations; no one is really to blame, yourself included.
As far as still loving him; when we love someone, it's a unique love. When you loose a lover, you loose their love. You miss the love you had with this person, and you may always do so at some point. Maybe simply trying to fondly hold those memories and accepting that these are all you'll have left of the love of this person instead of missing the person will help heal things with more time.