2D player jumps too high

Heres the script: using System.Collections; using System.Collections.Generic; using UnityEngine;

 public class PlayerScript : MonoBehaviour {
 
     public float speed;
     public float jumpSpeed;
     private Rigidbody2D rb;
     public bool isGrounded;
     public LayerMask ground;
     public Transform groundCheck;
     public Vector2 groundSize;
 
 
     void Awake () {
         rb = GetComponent<Rigidbody2D> ();
     }
     void FixedUpdate () {
         isGrounded = Physics2D.Raycast (transform.position, transform.TransformDirection (Vector3.down), 0.55f, ground);
         var mH = Input.GetAxis ("Horizontal");
         var movement = new Vector2 (mH, rb.velocity.y);
         rb.velocity = movement * speed;
         if (Input.GetKeyDown ("space") && isGrounded) {
             rb.velocity = new Vector2 (rb.velocity.x, jumpSpeed);
         }
     }
 }
 

When i try to jump my player jumps very very high. I tried setting the jumpSpeed variable more low, but the problem is when i set it too low it doesn't jump. my player is a square and i tried using rigidbody.AddForce but theres the same problem.
The velocity is set 5000, not to jumpSpeed
Can any body help?
Thank you