Dont destroy on load is destroying my Game Manager

using UnityEngine; using System.Collections; using UnityEngine.SceneManagement;

public class GameManager : MonoBehaviour {

 public static GameManager Instance
 {  
     get
     {
         if(_instance !=null)
         {
             return _instance;
         }
         else
         {
                 GameObject gameManager = new GameObject("GameManager");
                 _instance = gameManager.AddComponent <GameManager>();
                 return _instance;
             }
         }
     }
 private static GameManager _instance;
 public string titlescreenname = "TitleScene";

 [HideInInspector]
 public int PreviousScore = 0;



 public float pointsperunittravelled = 0.0f;
 private static float highScore = 0.0f;
 private bool hasSaved = false;
 private float Score = 0.0f;
 public float gamespeed = 10.0f;


 private bool gameOver =false;

 // Use this for initialization
 void Start ()
 {
                 
     if (_instance != this) 
     {
         if (_instance == null)
         {
             _instance = this;
         } 
         else
         {
             Destroy (gameObject);
         }
     }

     LoadHighScore ();
     DontDestroyOnLoad(gameObject);
     }
 
 // Update is called once per frame
 void Update () {
     if (SceneManager.GetActiveScene().name != titlescreenname) {
         
             if (GameObject.FindGameObjectWithTag ("Player") == null) {
                 gameOver = true;
             }
             if (gameOver) {
                 if (!hasSaved)
                 {
                     SaveHighScore ();
                     PreviousScore = (int)Score;
                     hasSaved = true;
                 }
                 if (Input.anyKeyDown) {
                     SceneManager.LoadScene(titlescreenname);
                 }
             }
             if (!gameOver) {
             
                 Score += pointsperunittravelled * gamespeed * Time.deltaTime;
                 if (Score > highScore) {
                     highScore = Score;
                 }
             }
     } 
     else {
         //Reset stuff for next game
         ResetGame();
     }
     }
 void ResetGame()
     {
     Score = 0.0f;
     gameOver = false;
     hasSaved = false;
     }
 void LoadHighScore()
     {
     highScore = PlayerPrefs.GetInt ("HighScore");
     }
 void SaveHighScore()
     {
     PlayerPrefs.SetInt ("HighScore", (int)highScore);
     PlayerPrefs.Save ();
     }
 void OnGUI()
         {
     if(SceneManager.GetActiveScene().name != titlescreenname)
     {
         int currentscore = (int)Score;
         int currenthighScore = (int)highScore;
         GUILayout.Label ("Score: " + currentscore.ToString ());
         GUILayout.Label ("HighScore: " + currenthighScore.ToString ());

             if(gameOver==true)
             {
                 GUILayout.Label("GameOver! PreesanyKey to reset!");
             }
         }
 
 }

}