Sorry but the OP's problem is that they have too much flour in the pan, nothing to do with the butter not being the right consistency or temperature. They just need to add more milk then recommended.
Melt butter/marg until runny; to piss about is called Al a Carte, which the French can keep thanks all the same.
Gradually add in some flour, using a whisk to create a paste or ball if you have added too much flour.
Slowly add milk, making sure to allow time for the flour to absorb the milk. Then add more milk until you have a smooth sauce.
Add the following you get -
Cheese - Cheese Sauce
Parsley - Parsley Sauce