Fixed Update Problems C# - Please help!

--- I Previously Asked A Question About This ---

• have my code for my player movement in script and i have been told to have the physics stuff in the update so that when i pause the game, the physics stuff pauses aswell*

Here is my code:

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 
 public class Movement : MonoBehaviour {
 
     private PauseMenuScript pauseMenuScript;
     private Fade myFadeScript;
 
     public float movementSpeed;
 
     public int jumpHeight;
     float jumpSpeed;
     Vector3 velocity;
     
     void Start(){
 
         pauseMenuScript = GameObject.Find ("Manager").GetComponent<PauseMenuScript> ();
 
         jumpSpeed = Mathf.Sqrt(-2*Physics.gravity.y*jumpHeight) + 0.1f;
 
     }
 
     void FixedUpdate(){
 
         rigidbody.AddRelativeForce(Vector3.right * 10 - rigidbody.velocity);
 
         if(Input.GetKeyDown("up") || (Input.GetMouseButtonDown(0)))
         {
             velocity = rigidbody.velocity;
             velocity.y = jumpSpeed;
             rigidbody.velocity = velocity;
         }
 
         //if (pauseMenuScript.isPaused)
 
     }
 
     IEnumerator WaitForTime (float waitTime){
 
         yield return new WaitForSeconds (waitTime);
     }
 
 }

When the game plays the key press does not always register so the game kinda feels bad, so how would i have it so that this dose not happen in the fixed update.

OR

How could i have it so that in the Update() function, i can reset the veolcity after the game has unpaused

Here is the first thread i posted about this to give you more context:

http://answers.unity3d.com/questions/900398/my-player-fly-across-the-screen-after-unpause-c.html

Thanks :D