When a person prays the prayer of salvation many times it suggests they have not grasped the issue of the gospel and the completed work of Christ for their sin. They are still trusting in themselves to some degree. People basically have a works mentality and simply trusting Christ for their salvation sounds too easy. They feel there just has to be something they must do rather than take God at His word. But no matter how hard a person works or tries, he can never qualify for salvation. Christ paid the penalty for us and we are to simply accept Him and His accomplishments on the cross, verified by the resurrection, for us as God’s gift for eternal life.
People lack assurance because they are not sufficiently trusting in or focused on Christ. They are looking to themselves for their assurance rather than to Christ. Complicating this is the problem of wrong expectations from self, probably because they do not see the change they expected. Read Paul’s comments in Romans 7:15f. This illustrates the struggle that all believers go through, but that the struggle is there, in no way means a person is not saved. Salvation does not remove the sinful nature. It breaks its hold so we do not have to yield to it, but that is a choice and a matter of growing in our faith. Faith needs to grow and it only grows through the knowledge of Scripture (biblical understanding) and time.
While some change takes place immediately in a Christian’s life, real spiritual change and maturity is a life-long process that requires time and growth in the knowledge of the Word. Some people may experience more change immediately than others, but the lack of change does not mean you are not saved. If you have believed that you are a sinner, that Christ died for you, then the lack of change is caused by the need of biblical understanding, growth and time. But, we are never to look to ourselves for assurance. If we do, we will never have assurance. Instead, we are to look to Christ.
Romans 8:32-39 He who did not spare His own Son, but delivered Him up for us all, how will He not also with Him freely give us all things? 33 Who will bring a charge against God’s elect? God is the one who justifies; 34 who is the one who condemns? Christ Jesus is He who died, yes, rather who was raised, who is at the right hand of God, who also intercedes for us. 35 Who shall separate us from the love of Christ? Shall tribulation, or distress, or persecution, or famine, or nakedness, or peril, or sword? 36 Just as it is written, “For Thy sake we are being put to death all day long; We were considered as sheep to be slaughtered.” 37 But in all these things we overwhelmingly conquer through Him who loved us. 38 For I am convinced that neither death, nor life, nor angels, nor principalities, nor things present, nor things to come, nor powers, 39 nor height, nor depth, nor any other created thing, shall be able to separate us from the love of God, which is in Christ Jesus our Lord. (NASB)