So, when did you add the sugar.  As dental Assistant's recipe shows, you want to add the sugar before/when you are boiling. 

Butter tends to be more firm than margarine, so I don't think the butter is the issue.

I tend to like Peanut Butter Fudge.  Looking at Dental Assistant's recipe, it is tempting to try it. However, having diabetes, I keep telling myself "No, I shouldn't."  So far, I am resisting...... however, tempting it is.