[Fatalivy]

I've had Ivy since she was a baby and began obedience training with her with a personal in home trainer at about 5 months old. She worked with Ivy on sit and down commands, and to this day, Ivy will sit, but she sits or does down out of fear. When I approach her, she sits and starts shivering. Sometimes, when I say sit, she goes to down and hides her face. She has ALWAYS been this way and I've never laid a hand on her, so I'm not sure why she is like this.
Yesterday, she was in the backyard playing with my other two dogs and when I say inside, they all usually come running. Well, yesterday, she ran under the steps and was shivering and would not come out. I had to go and get her from under the steps.

Today I decided to start over with sit and down commands. She started shaking like crazy and won't even look at me. I tried for 5 mins and gave up. She is not treat movitvated although we have always used treats to try and teach her. What else can I try and why do you think she's so fearful?