... and I was thinking if they turn out hard again, it might be that the marshmallows are being cooked too long or over too high of heat....
I don't make these in particular, but I've been cooking for a long time. If you heat the sugar (in the marshmallows) too high and/or too long it will become hard. The way to make hard candy like lollipops is to heat the sugar to 300 degrees F and then let it cool.
You'll want to heat the marshmallows and butter on low heat (or double boiler) just until melted.