The difference between stubborness and fortitude is the intent. From a behavioural and external point of view, both stubbornness and fortitude look like the same thing. However, Catholic morality isn't only about the external behaviour, it is about the intent behind it.
Stubbornness is what we call someone who refuses to change their mind when they are wrong. Steadfastness on the other hand is what we call someone who stays in the right no matter the consequences.
The only way to be steadfast is to have following God's will as the intent. All virtues come from God, whereas the Devil wants to use our temperament against us so that we go to hell.
Being called "stubborn" by heretics is not a bad thing.