Stuff

 Vector3 moveVector = new Vector3(0, 0, 0);
            KeyboardState keyState = Keyboard.GetState();
            if (keyState.IsKeyDown(Keys.Escape))
                this.Exit();

            if (keyState.IsKeyDown(Keys.Up) || keyState.IsKeyDown(Keys.W))
                moveVector += new Vector3(0, 0, -1);
            if (keyState.IsKeyDown(Keys.Down) || keyState.IsKeyDown(Keys.S))
                moveVector += new Vector3(0, 0, 1);
            if (keyState.IsKeyDown(Keys.Right) || keyState.IsKeyDown(Keys.D))
                moveVector += new Vector3(1, 0, 0);
            if (keyState.IsKeyDown(Keys.Left) || keyState.IsKeyDown(Keys.A))
                moveVector += new Vector3(-1, 0, 0);
            if (keyState.IsKeyDown(Keys.Q))
                moveVector += new Vector3(0, 1, 0);
            if (keyState.IsKeyDown(Keys.Z))
                moveVector += new Vector3(0, -1, 0);
            Matrix cameraRotationNonFly = Matrix.CreateRotationY(leftrightRot);
            Matrix cameraRotation = Matrix.CreateRotationX(updownRot) * Matrix.CreateRotationY(leftrightRot);

            Vector3 rotatedVector = Vector3.Transform(moveVector, cameraRotationNonFly); // fly mode.
            //Vector3 rotatedVector = Vector3.Transform(moveVector, cameraRotationNonFly);
            float amount = (float)gameTime.ElapsedGameTime.TotalMilliseconds / 1000.0f;

            cameraPosition += moveSpeed * rotatedVector * amount;

           
            if (currentMouseState != originalMouseState)
            {
                
                float xDifference = currentMouseState.X - originalMouseState.X;
                float yDifference = currentMouseState.Y - originalMouseState.Y;
                
                    leftrightRot -= rotationSpeed * xDifference * amount;
                    updownRot -= rotationSpeed * yDifference * amount;
                    Mouse.SetPosition(GraphicsDevice.Viewport.Width / 2, GraphicsDevice.Viewport.Height / 2);
                
            }

         
            Vector3 cameraOriginalTarget = new Vector3(0, 90, -90);
            Vector3 cameraRotatedTarget = Vector3.Transform(cameraOriginalTarget, cameraRotation);
            Vector3 cameraFinalTarget = cameraPosition + cameraRotatedTarget;
            Vector3 cameraLookat = cameraPosition + cameraRotatedTarget;

            Vector3 cameraOriginalUpVector = new Vector3(0, 1, 0);
            Vector3 cameraRotatedUpVector = Vector3.Transform(cameraOriginalUpVector, cameraRotation);

            view = Matrix.CreateLookAt(cameraPosition, cameraLookat, cameraRotatedUpVector);
            originalMouseState = Mouse.GetState();