on forgiving

why is forgiving so flippin hard?
when someone does something that hurts me i can always pretty much forgive them straight away. Either I am able to see it from their perspective, or i understand where they're coming from, and so i can extend forgiveness without that much trouble. This time i'm having a really hard time with it.
I think it's when one particular action makes you realise the status of a whole relationship-- one text message exposes an entire "friendship" as manipulative and one sided. What do you do at this point?
when it's assumed that you will forgive straight away, and "matt can handle it", so people dont spare a second thought to how they treat you. At some point you stand up and say- guess what? You're not manipulating me anymore. I'm not gonna be taken for granted, because i'm a real person and i'm not here to cater to you.
Is that wrong? I have a sneaking feeling that it is wrong. But i'm having trouble getting past that point into forgiveness right now.