This argument started out quite sadly, in all fairness.

Sam had accidentally referred to you as “Jess”, his late girlfriend’s name, which causes you to spiral into a dark mindset of “am I just a replacement? Am I not my own person in Sam’s eyes? Is he only with me because he sees her in me?”

Which is why you now find yourself upset with Sam as he tries to explain and reason with you, however fruitless that seems at the moment.