Get a constant Speed

Hi i have a object that go forward and jump in the air and my x axis is slowing when i set flap at true or when i get in contact with another object. Is there possible to get a constant x speed so my GameObject won't slow down?

Here is my code:

 Vector3 velocity = Vector3.zero;
 public Vector3 flapVelocity;
 public float flapSpeed = 1.2f;
 public float ForwardSpeed = 1f;
 bool flap = false;
 bool reverse = false;
 bool changeGravity = false;
 // Use this for initialization
 void Start()
 {


 }
 //Graphiques et mises à jour d'inputs
 void Update()
 {
     if (Input.GetKeyDown("space") || Input.GetMouseButtonDown(0))
     {
         flap = true;
     }
     else if (Input.GetKeyDown(KeyCode.LeftShift) || Input.GetMouseButtonDown(0))
     {
         if (reverse == false)
         {
             reverse = true;
         }
         else
         {
             reverse = false;
         }
         changeGravity = true;
     }
 }
 // Mises à jour des moteurs physiques
 void FixedUpdate()
 {
     Rigidbody2D saut = GetComponent<Rigidbody2D>();
     if (changeGravity == true)
     {
         if (reverse == true)
         {
             GetComponent<Rigidbody2D>().gravityScale = -1.8f;
             changeGravity = false;
         }
         else if (reverse == false)
         {
             GetComponent<Rigidbody2D>().gravityScale = 1.8f;
             changeGravity = false;
         }
         changeGravity = false;
     }
     if (flap == true)
     {
         if (reverse == true)
         {
             GetComponent<Rigidbody2D>().gravityScale = -0.8f;
             GetComponent<Rigidbody2D>().velocity = -(flapVelocity) / 2;
         }
         else if (reverse == false)
         {
             GetComponent<Rigidbody2D>().gravityScale = 0.8f;
             GetComponent<Rigidbody2D>().velocity = flapVelocity / 2;
         }
         //GetComponent<Rigidbody2D>().AddForce(Vector2.up * flapSpeed);
         GetComponent<Rigidbody2D>().AddForce(Vector2.right * ForwardSpeed);
         flap = false;
     }
     transform.Rotate(0, 0, -150 * Time.deltaTime);
 }