Assurance of salvation and eternal security are not identical concepts. The former describes my level of confidence in my relationship with God, while the latter refers to what God has actually done, whether I fully understand it and benefit from that knowledge or whether I quiver in terror of eternal damnation every time I catch myself sinning yet again.
Which I will, and so will you. One may feel confidence with no scriptural basis. One may also feel fear for which there is no biblical reason.