Question Slopes are causing my rigidbody to gain velocity. How can I prevent this?

This is my script:

Code (CSharp):

1.    private void Update()
2.     {
3.         if (Input.touchCount > 0)
4.         {
5.             Touch touch = Input.GetTouch(0);
6.  
7.             switch (touch.phase)
8.             {
9.                 case TouchPhase.Began:
10.                     startPosition = touch.position;
11.                     break;
12.                 case TouchPhase.Moved:
13.                     // The force on the x axis is calculated...
14.                     // by how far away the finger is from the starting point
15.                     xForce = touch.position.x - startPosition.x;
16.  
17.                     // Limit x-axis force to prevent advantages from longer screens
18.                     if (xForce > maxXForce)
19.                     {
20.                         xForce = maxXForce;
21.                     }
22.                     else if (xForce < -maxXForce)
23.                     {
24.                         xForce = -maxXForce;
25.                     }
26.                     break;
27.             }
28.  
29.             fingerDown = true;
30.         }
31.         else
32.         {
33.             fingerDown = false;
34.         }
35.     }
36.  
37.     private bool OnGround()
38.     {
39.         Vector3 rayOrigin = transform.position;
40.         float rayDistance = 1.0f;
41.         LayerMask layerMask = LayerMask.GetMask("Ground");
42.         RaycastHit hit;
43.         if (Physics.Raycast(rayOrigin, Vector3.down, out hit, rayDistance, layerMask))
44.         {
45.             slopeNormal = hit.normal;
46.             // Calculates the angle along z axis
47.             slopeAngle = Vector2.Angle(Vector3.up, slopeNormal);
48.  
49.             return true;
50.         }
51.  
52.         return false;
53.     }
54.  
55.  
56.     private void FixedUpdate()
57.     {
58.         if (fingerDown)
59.         {
60.             // Calculations to make it more responsive
61.  
62.             // calculate the direction we want to move in and our desired velocity
63.             float targetSpeed = (xForce * sideSpeed);
64.             // calculate the difference between current velocity and desired velocity
65.             float speedDif = targetSpeed - rb.velocity.x;
66.             // Change acceleration rate depending on situation
67.             float accelRate = (Mathf.Abs(targetSpeed) > 0.01f) ? acceleration : decceleration;
68.             // Applies acceleration to speed difference, then raises to a set power so acceleration increases with higher speeds
69.             // finally multiplies by sign to reapply direction
70.             float movement = Mathf.Pow(Mathf.Abs(speedDif) * accelRate, velPower) * Mathf.Sign(speedDif);
71.  
72.  
73.             // Making the ball move along a slope
74.            
75.             if(OnGround())
76.             {
77.                 forceDirection = Vector3.ProjectOnPlane(new Vector3(movement, 0, 0), slopeNormal);
78.             }
79.             else
80.             {
81.                 forceDirection = new Vector3(movement, 0, 0);
82.             }
83.  
84.             // Makes the ball move from left ot right
85.             rb.AddForce(new Vector3(forceDirection.x, forceDirection.y, 0));
86.  
87.         }
88.  
89.         // Makes the ball move forward
90.         rb.AddForce(Vector3.forward * forwardSpeed * Time.fixedDeltaTime);
91.     }