Animations Only Playing Once Per Key Press

I'm trying to animate the player character for a Unity game I'm working on. I have the different sprites for the animation, a sprite renderer for the player, and an animation program. However, when I run the game, if I press and hold down the move key, the player character will change its sprite to the next sprite in the animation and then stop changing sprites. If I release and then press the key down again, it will change its sprite, and again and again.

Basically, if I have Sprites 1-7 to cycle through for the animation, when I press and hold the move key down, the player will switch its sprite to Sprite 2, but not to any of the others. If I release the move key and then press it again, it will cycle to Sprite 3. If I release the move key and press it again, the player character will cycle to Sprite 4. If I hold the move key down w$$anonymous$$le the player character is on Sprite 4, it will not cycle to Sprite 5. T$$anonymous$$s is the only way it can cycle through the animation sprites. What am I doing wrong and how can I fix the issue so that the player character will fluidly cycle through the animation as long as the move key is pressed?

Here is the code for the animation:

 using UnityEngine;
 using System.Collections;
 
 public class HorseAnim : MonoBehaviour {
 
     public Sprite[] spritesRight;
     //public Sprite[] spritesIdle;
     public float framesPerSecond;
     private SpriteRenderer spriteRendererRight;
 
 
     // Use t$$anonymous$$s for initialization
     void Start () {
         spriteRendererRight  = renderer as SpriteRenderer;
 
     }
     
     // Update is called once per frame
     void Update () {
         w$$anonymous$$le(Input.GetButtonDown("Horizontal"))
         {
         int index = (int)(Time.timeSinceLevelLoad * framesPerSecond);
         index = index % spritesRight.Length;
         spriteRendererRight.sprite = spritesRight[ index ];
             break;
         }
     }
 }