I made marshmallow fondant yesterday and kneaded it for an hour and it still is a little too soft, it isn't pulling like taffy, it is breaking. Should I just keep adding more powdered sugar? Or should I add cornstarch?

It sounds to me like it needs more marshmallows and a dab of Crisco.   You could use the trick Liz Marek uses for her famous marshmallow fondant, add about a pound of Wilton (or whatever you have handy) fondant.  She discovered this trick by accident and it pulls everything together into a gorgeous fondant.

I'm sorry i should have mentioned in post that the recipe I am using is the liz Marek Marshmallow fondant.

I've made it before with no problems. I'm thinking may be the humidity