On the right rein my 6-years-old horse falls to the inside but when i use my leg to correct it she gets stressed and starts to freak ouy. Do you jave any tips to help him understand?

Hello Lian,
Maybe you use your leg too strongly?
I would suggest you to first teach you horse respond a soft pressure on ground work, ask him to move his shoulders and hip when you ask him. This way he will understand that a pressure on his body won't hurt him.
Then when you ride him, be careful to push him first with a calf pressure than directly with your heel.
Be patient, ask few at the begining and congratulate him when he understands.
Good luck! :)