While I know its difficult that Sam has not been able to return your affection nor congratulate you on your upcoming nuptials, I suspect it may be because he has mixed emotions too. If he is of a monogamous mindset, he may be afraid to acknowledge any feelings he has for you, and at the same time may be jealous of your fiancÚ - hence no congratulations. So it may not in fact be rejection, but just someone who doesn't cope well with his own emotions. If this is the case, although it may "feel" like rejection; it's not. If possible, I would attempt to reframe it.
An additional strategy might be to simply ask Sam why he has not expressed congratulations to you. Maybe he's been self-absorbed and needs to pull his head out.
As for loving him, while it's difficult to do, I would suggest simply accepting that you do, that it's unlikely to change, and there's nothing you can do about it. A lot of tough emotions are easier to deal with if we give ourselves permission to feel them without judgment or the need to "fix" them. (And while I know this, I still struggle with it.