negating a force 2D

Hi! I'm working on a rocket launcher script, and I'm having this problem: I fire the rocket, there is an initial "warmup" period where the rocket flies up, and after that I want it to go to its target (the mouse pos), however, the force from the instantiation period offsets the rocket, and it becomes inaccurate - how can I fix this, while still using rigidbodies?

thx for any help!

ps: ik the code is probably a mess

 {
     StartCoroutine(timer());
     rigidbody = GetComponent<Rigidbody2D>();
     Destroy(gameObject, 5);
     var mousePos = Camera.main.ScreenToWorldPoint(Input.mousePosition);
     mouseDir = mousePos - gameObject.transform.position;
     mouseDir.z = 0.0f;
     mouseDir = mouseDir.normalized;
     trailRenderer.enabled = false;
     transform.parent = parent.transform;
 }

 void Update()
 {
     if (lauch)
     {
         rigidbody.AddForce(mouseDir * MoveSpeed);

         Vector2 v = rigidbody.velocity;
         float angle = Mathf.Atan2(v.y, v.x) * Mathf.Rad2Deg;
         transform.rotation = Quaternion.Lerp(transform.rotation, Quaternion.AngleAxis(angle, Vector3.forward), Time.deltaTime * lerpTime);

         trailRenderer.enabled = true;
         transform.parent = null;
     }
     else
     {
         rigidbody.AddForce(transform.up * MoveSpeed2);
     }
 }