Player jump double jump double speed

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class PlayerController : MonoBehaviour
{
    private Rigidbody playerRB;
    private Animator playerAnim;

    public ParticleSystem explosionParticle;
    public ParticleSystem dirtParticle;

    public float jumpForce = 10;
    public float gravityModifier;

    public float doubleJumpForce;
    public bool doubleJumpUsed = false;
    public bool doubleSpeed = false;

    public bool isOnGround;
    public bool gameOver = false;

    public AudioClip jumpSound;
    public AudioClip crashSound;
    private AudioSource playerAudio;

    // Start is called before the first frame update
    void Start()
    {
        playerRB = GetComponent<Rigidbody>();
        Physics.gravity *= gravityModifier;
        playerAnim = GetComponent<Animator>();
        playerAudio = GetComponent<AudioSource>();
    }

    // Update is called once per frame
    void Update()
    {// press space to make player jump but not to jump when in the air and not jump when dead
        // and dirt stops when jumping
        if (Input.GetKeyDown(KeyCode.Space) && isOnGround && !gameOver)
        {
            playerRB.AddForce(Vector3.up * jumpForce, ForceMode.Impulse);
            isOnGround = false;
            playerAnim.SetTrigger("Jump_trig");
            dirtParticle.Stop();
            playerAudio.PlayOneShot(jumpSound, 1.0f);
            //double jump (to line 49)
            doubleJumpUsed = false;
        }
        else if(Input.GetKeyDown(KeyCode.Space) && !isOnGround && !doubleJumpUsed)
        {
            doubleJumpUsed = true;
            playerRB.AddForce(Vector3.up * doubleJumpForce, ForceMode.Impulse);
            playerAnim.Play("Running_Jump", 3, 0f);
            playerAudio.PlayOneShot(jumpSound, 1.0f);
        }

        if(Input.GetKey(KeyCode.LeftShift) && isOnGround)
        {
            doubleSpeed = true;
            playerAnim.SetFloat("Speed_Multiplier", 2.0f);
        }
        else if (doubleSpeed)
        {
            doubleSpeed = false;
            playerAnim.SetFloat("Speed_Multiplier", 1.0f);
        }
    }
    //when player collides with the ground is on ground is true and dirt is kicked up
    private void OnCollisionEnter(Collision collision)
    {
        if (collision.gameObject.CompareTag("Ground"))
        {
        isOnGround = true;
            dirtParticle.Play();

        } //if player hits obstacle then game over and death animation plays and he cant jump
        // and dirt stops coming out of his feet
        else if (collision.gameObject.CompareTag("Obstacle"))
        {
            gameOver = true;
            Debug.Log("Game Over");
            playerAnim.SetBool("Death_b", true);
            playerAnim.SetInteger("DeathType_int", 1);
            explosionParticle.Play();
            dirtParticle.Stop();
            playerAudio.PlayOneShot(crashSound, 1.0f);

        }
    }
}