How do you switch from Western forehand grip to a Semi western grip?

Question

My hand automatically when i grip my racquet and does a forehand i always move my grip to the western even though i dont like it, i want to know how to make my body to accept a semi western grip, but i try to move my have to the semi western before i do my forehand, it always slide to the western forehand grip at the last minute. i dont like this ]: HELP!!!