My fiancee is too sensitive and I am going to tell her.
What do you think will happen when you tell her she's sensitive?
She'll be more open about her emotions
You may mean well, but like you said she's sensitive. She might not take things well and think too hard of it, which is counterproductive. You need to approach it in a kinder way so she won't get distressed.
I think I'll get mad and tell her she should be tougher.
You can do this thing gently without being so mean. Just tell her you'd like to help her not be so sensitive.