First of all, congratulations on your engagement.
Second of all, it's great that you are in a place where you think it is safe to tell her. As for how to tell her, it was really hard to get the words out for me, but I managed to tell her I was sexually assaulted. You have your own words that will work to you... just be clear and direct. Tell her during a time when she's not stressed out about something else and not busy doing other things. The day I told my wife, I had someone else watch our son and I made sure I told her after she had her coffee and got a chance to work out. People are usually in a better place after having something to eat, too.
As for a reaction, I would hope that she would be supportive of you and your need to heal from your assault. You can't predict how she'll react but in my experience women are easier to tell than men... hopefully she will see that it wasn't your fault. When I told my wife, she threw a microwave onto the kitchen floor and also shattered the pitcher that came with our blender. Unfortunately this is not the first time she has done that, so... hopefully your woman is in better control of her emotions than mine is. but I can say she was angry at the man who did it, not at me.
Sometimes women will suspect that something happened and I have heard stories where she isn't necessarily shocked. I hope she will give you the support you need and deserve.
“If a man wishes to be sure of the road he treads on, he must close his eyes and walk in the dark.”
- Saint John of the Cross