As the believers in Christ, we can and should have the assurance of our eternal salvation; praise the Lord for that! However, we cannot have the assurance we will reign with Christ during the Millennium; even the Apostle Paul was not assured of that (1 Cor. 9:24-10:5).
In fact, we will not know whether that 1,000 years will be a time of reward for us, or a time of discipline, until we appear before the judgment seat of Christ. At that time we will be judged according to our works after we were saved (2 Cor. 5:10).