That's not how it works in the real world. You can't start a romantic relationship with someone under the guise of being friends, it just doesn't work out that way very often. In her mind, you were friends and she has no romantic interest in you. Then you confess that you love her while she already had a boyfriend. That was very inappropriate.

She is now thinking back over every interaction that you both had and re-evaluating your actions and words in the context of someone who was secretly trying to get into her pants the whole time. I'm not saying that's what you were doing, but you dropped a bombshell on her and she feels betrayed because you were being duplicitous.

Also, love doesn't happen overnight, so you obviously have had some amount of feelings for her for a long time. It would have been much more appropriate to tell her about your feelings back when you only sort of liked her.