My fudge is getting thick when I start to stir in marshmallows, chocolate chips and nuts
Most likely you need to turn up the temperature a little before adding the new ingredients. All those ingredients are about room temperature, and there are a fairly large quantity of them compared to the mixture in the pan. This means that they cool down the mixture considerably when added. The cooling means that the temperature is not sufficient to melt marshmallows any more, and the fudge begins to set (thicken). Increasing the temperature slightly will help the marshmallows still melt.