Destroying Prefab Also Destroys Prefab Reference

Whenever I shoot a bullet in my scene, my script replaces the prefab reference of the bullet for bullet (clone). Eventually it becomes something like bullet (clone) (clone) (clone) (clone) (clone). When the most recently created bullet is destroyed, the reference is destroyed with it. With no bullet reference in the script, I can't shoot, and I get an error that says MissingReferenceException.

I don't know what the problem is, and I tried looking it up but the only answer was in JS, which I can't read.

I'm speculating that the problem is I am instantiating the prefab ITSELF, not a clone of the prefab. Unfortunately I have no idea how to change that, and I'm not positive that's the problem.

 using UnityEngine;
 using System.Collections;
 
 public class PlayerShot : MonoBehaviour {
     public GameObject projectilePrefab;
     private float timeclicked;
     public Transform thispos;
     public Transform thisposr;
     public float speed = 1.5f;
     public Vector3 target;
     public static int leftshot;
     
     void Start () {
         target = transform.position;
     }
     
     void Update () {
         if (Input.GetKey (KeyCode.Space)&& Time.time > timeclicked && PlayerController.left == false && Time.time > PlayerController.jumptime) {
 
             projectilePrefab = Instantiate(projectilePrefab, thispos.position, thispos.rotation) as GameObject;
             timeclicked = Time.time + 0.2f;
             leftshot = 1;
         }
         if (Input.GetKey (KeyCode.Space)&& Time.time > timeclicked && PlayerController.left == true && Time.time > PlayerController.jumptime) {
         
             projectilePrefab = Instantiate(projectilePrefab, thisposr.position, thisposr.rotation) as GameObject;
             timeclicked = Time.time + 0.2f;
             leftshot = -1;
         }
     }   
 }
 
 using UnityEngine;
 using System.Collections;
 
 public class SpeechProj : MonoBehaviour {
         private Vector3 shotTarget;
     public Rigidbody2D thisrigid;
     public Vector3 target;
     public float xGo;
         public float yGo;
     void Start () 
     {
         rigidbody2D.AddForce (Vector3.right * 1200 * PlayerShot.leftshot);
         target = Camera.main.ScreenToWorldPoint(Input.mousePosition);
     
     }
 
     // Update is called once per frame
     void Update () 
     {
         }
         void OnCollisionEnter2D(Collision2D coll) 
         {
             if (coll.gameObject.tag == "Enemy" || coll.gameObject.tag == "Blocker")
             {
             Destroy(this.gameObject);
             }
     }
 }