IMHO, struggling for assurance is a healthy problem to have. It shows that one is looking for evidences and fruit and genuine assurance that a supernatural work has been done.
The Gospel of John compares the work of the Holy Spirit in regeneration to wind where you cannot tell where it comes from and where it goes (John 3:8). Like the wind, assurance can be elusive.
The worse problem is when those who are spiritually lost are thinking that they have already been saved. In recent decades, too many have been falsely assured by human formulas and human reassurances rather than by the supernatural work of genuine spiritual regeneration in the new birth.
Looking at the letters to the angel (messenger) at each of seven churches (Revelation 2-3), there are promises that are given in each letter that apply only to him that overcomes. Even to first century churches, warning was given to sustain in Christian living and to overcome. He that hath an ear, let him hear with the Spirit says to the churches.
Assurance is possible:
JC Ryle is one author who has written on the subject of gaining full assurance. There are others.
