Ending a relationship is difficult no matter who you are or what the circumstances. If you were already unhappy before the marriage, then who is to say that by getting married would have changed that.
Why does it hurt so much, because you are human and have feelings obviously. If it didn't hurt then one would think that you didn't have a heart to start with.
Why is all the blame placed on you? Probably because his defenses have gone up and he is trying to protect himself and it is easier to blame others for your problems then to accept that you had anything to do with them in the first place.
You need to keep in mind that you are not the bad person here, and that you are only doing what you think is the right thing. If you had gone through with the wedding and things never got any better then where would you be, in divorce court. I think you made the right choice by calling it off before it ended up there. Maybe you can work things out and be able to come to a better place together and then discuss getting married again in the future. Best of luck to you and stay strong.