The point of this thread is that those who are regenerate will rejoice in the truth and be glad. All others....read the last few lines.
John 15:18-25 is a difficult passage BECAUSE it can and often does cause a false sense of security. Not only do the righteous use it, the false-teacher does as well. He/she deems that it supports their own views against Christendom. Because of that, we must weigh such notions heavily against other scriptures that call us to examine the truth.
I'm not sure if what I say will directly speak to you, but I lift it up in hopes it may.
It is true that the Spirit will attest to our Spirit that we are His, Romans 8:16 but He doesn't always tell me I'm "right." Sometimes our God tells me to seek the truth of a matter out in the scriptures and other times, 2 Corinthians 13:5 1 Timothy 4:16 to correct my thinking by them. 1 Thessalonians 5:21 Otherwise I think we'd be too prideful. We aren't capable of that in this old body of flesh, imho. My greatest revelation from God was that God didn't think like me and was not a man. This necessarily meant and means that I have to renew my mind. Romans 12:2 Ephesians 4:22-24 I no longer confuse assurance that we are His with 'having it all correct.' Paul says we see through a glass darkly 1 Corinthians 13:12,13 and John tells us it is only when we 'see Him, we will be like Him for we will see Him as He is." 1 John 3:1-3
I believe we are in this position 1) because we are finite creatures, 2) because sin affects our minds and thus we are always battling against getting it wrong, and 3) that we'd be humble and dependent on God throughout our lives.