Why does it feels so bad when someone betrayed you

It's because the lesson that some parts of you immediately (and rightly) take from betrayal is that you are not nearly as good at understanding and dealing with other people as you thought you were, and that these shortcomings need to be fixed immediately if you don't want to keep having more bad experiences like that from now on.

Hence a civil war rages in your mind, in which some parts of you are screaming, "Look at how bad you fucked up! Obviously this person wasn't anything like what you thought they were, how is it even possible for you to be that wrong about something and not notice it at all until now?" These parts are then in full-on purge mode, hunting around for the other parts of you that need to be destroyed, subdued, or modified in response to this scandal.

Other parts are shocked at their own failure and will alternate between crippling guilt for plunging the entire human organism they exist within into a crisis because of their own mismanagement of things; seething anger or sadness at the betrayer as the proximate catalyst for all of this in the first place; and desperately trying to contextualize the whole thing in a way that makes it not their fault and justifies them continuing to think and act in the way they like and are used to.

I don't know the science behind that, but that's how it goes.

Unfortunately that’s the way it’s supposed to feel

It makes you aware of exactly how vulnerable you are.

It's prob tied to our ego and our expectations. There are many forms of betrayal

I'm living that right now. Worse as this was like a 4th chance or something. I'm more mad and disappointed in myself.

I wish I knew why. Should have seen it coming that way the hurt might be little less intense.

That overrated feeling known as 'trust'

Because you trusted that person, being backstabbed is horrible.

Cuz they betrayed you