Need help with a "roll" script (C#)

I'm trying to make a simple 3rd person RPG and want the character to be able to do a roll to dodge. Right now it just makes it impossible to walk in the opposite direction after hitting Shift-W. My current code is this: public class Roll : MonoBehaviour {

     public float testStamina = 100.0f;
     public float coolDown = 2.0f;
     public float rollLength = 5.0f;
     public float lastRoll;
     public float rollDuration = 2.0f;
     public float startTime;
     bool canRoll;
     
     // Use this for initialization
     void Start () {
         lastRoll = Time.time;
         canRoll = true;
     }
     
     // Update is called once per frame
     void Update () {
         if(coolDown >= Time.time - lastRoll)
             canRoll = true;
         if(Input.GetKey(KeyCode.W) && Input.GetKey(KeyCode.LeftShift) && canRoll == true){
             canRoll = false;
             startTime = Time.time;
             if(startTime - rollDuration <= Time.time){
                 rigidbody.AddForce(Vector3.forward * -100f);
             }
             lastRoll = Time.time;
         }
     }
 }

I feel like its something I should really know how to do, but its 2 in the morning and I need to finish this. Any help is greatly appreciated.