Scripture makes it pretty clear that our assurance is based on the presence of spiritual fruit in our lives as well as a pattern of spiritual growth and increasing maturity:

For land that has drunk the rain that often falls on it, and produces a crop useful to those for whose sake it is cultivated, receives a blessing from God. But if it bears thorns and thistles, it is worthless and near to being cursed, and its end is to be burned. Though we speak in this way, yet in your case, beloved, we feel sure of better things—things that belong to salvation. (Hebrews 6:7-9, ESV, emphasis added)

But what are these fruits? That also is made clear:

His divine power has granted to us all things that pertain to life and godliness, through the knowledge of him who called us to his own glory and excellence, by which he has granted to us his precious and very great promises, so that through them you may become partakers of the divine nature, having escaped from the corruption that is in the world because of sinful desire. For this very reason, make every effort to supplement your faith with virtue, and virtue with knowledge, and knowledge with self-control, and self-control with steadfastness, and steadfastness with godliness, and godliness with brotherly affection, and brotherly affection with love. For if these qualities are yours and are increasing, they keep you from being ineffective or unfruitful in the knowledge of our Lord Jesus Christ. For whoever lacks these qualities is so nearsighted that he is blind, having forgotten that he was cleansed from his former sins. Therefore, brothers, be all the more diligent to make your calling and election sure, for if you practice these qualities you will never fall. For in this way there will be richly provided for you an entrance into the eternal kingdom of our Lord and Savior Jesus Christ. (2 Peter 1:3-11, ESV, emphasis added)

Unless I’m misinterpreting this passage, the text does two things, in general: 1) It commands us to make our calling and election sure (i.e., to seek assurance), and 2) it tells us how to go about obtaining that certainty. Since what Peter described here is a process, it seems clear that gaining such certainty is not instantaneous. It is acquired only after developing a pattern of increasing holiness, Christlikeness and maturity.

I cannot imagine that God would give anyone assurance of salvation who is stagnating in their Christian life, failing to grow or, even worse, taking steps backward. There is only one proper direction for the true Christian: forward into more and more Christlikeness. If that forward progression is not happening, we have good reason to doubt our salvation.