I find that for me there is a difference between forgiving someone and continuing to trust them. For me in particular, it's fairly easy for me to forgive someone but I've never quite figured out why. However, if someone hurts me deeply enough, it is very difficult for them to regain the trust they once had with me.
Fortunately for me, even though I have a great memory for facts, I have a difficult time staying grounded in the present moment and thus my memory suffers from this as well. If I don't want to remember something, I simply bury it in my memory until I have a hard time remembering the exact details. But in the rare case where I can't forget something, if an instance where I've been hurt comes to mind and I don't have an agonizing feeling in the pit of my stomach concerning it, then I figure I already forgave the person and move on with my life. If I haven't, I figure out how to get there and then move on. To me, some things aren't worth getting too hurt over.
as far as the original topic goes, I'm not sure if I would agree that "failure to forgive" would be an emotional disorder or not since I've never really thought about it either way before. I suppose it's possible since from my experience failing to forgive someone only eats away at my soul until I retaliate toward someone else. This happened to me some in the past and trust me, it wasn't pretty.