When do you think it's the gate from just being careful to being a yandere?
I don't understand the question due to formatting. Are you asking when it turns to simply being a watchful/mistrustful person in love to being a full on yandere?
I would like to know from you where exactly when does she stop becoming just a jealous person and become an actual yandere?
You'd have to define, exactly, what the word "yandere" means.