I need help getting confidence to do my roundoff back handspring by my self.?

i allways like finnish my roundoff and jump back and feak out and just jump straight onto my back. how do i get the comfidence to do it. or at least how do i do it just jump back and lock out my arms because ive done it 2 times by myself and i landed on my knees and i can do it with a spot but the past few days i have freaked out and got hurt every time i tried by myself.