Vector3 Position X animation

Question

I want to make my vector go right till it hits 300 X and then come back to the point where it started smoothly.
The code I have makes my vector to go smoothly to the right but I cant figure it out why its not going back....

Here is the code :) 

        public override void Update(ref GameTime pGameTime)

        {

            Vector3 tokenPosition = this.PossessedToken.Position;

            tokenPosition.X = tokenPosition.X + 1;

            this.PossessedToken.Position = tokenPosition;

            if (tokenPosition.X > 300)

            {

                Vector3 tokenPosition1 = this.PossessedToken.Position;

                tokenPosition1.X = tokenPosition1.X - 1;

                this.PossessedToken.Position = tokenPosition1;

            }

        }        

        #endregion