In 23 years as a Christian, I have struggled at times with assurance of salvation. In my personal experience, it's so easy to slip into doubting my salvation because (at least in my part of the world) there is no real cost for believing—that is, there is no persecution. Therefore it's hard to see the difference between people who merely give mental assent to a series of facts about Christ and those who have an implicit trust that characterizes saving faith and produces fruit.
However, in many other locales there is a very high cost: loss of job, loss of family, even loss of life. In these cases there is often a huge difference between bare mental assent and a public declaration that "Jesus is Lord".
The point is that it may be easier for a person to doubt his or her salvation in a place where there is little or no persecution.
But I believe it is possible for Christians anywhere and at any time in history to doubt their salvation, and yet God is sovereign over all. He put us where we are (Acts 17:26) and knows our struggles. Someone can believe in God, believe what the Bible says about Christ is true, and give his or her life to Jesus. But when we sin as we all inevitably do, like John Piper has said, (1) some of us slide into hypocrisy and presumptuous sin, and also (2) some of us doubt our salvation because we don't think our fruit will ever be good enough to demonstrate that we are Christians (I have been in both categories).
Like many others, I have also thought of 1 John 3:9. I didn't “brush off” the meaning of this verse, and I feared that the "practice of sinning" indicated that I was not saved. I certainly practiced sinning like the verse says, so for a while I examined the "how" of my salvation experience and attempted to understand those things that really indicated that I was saved. In the process I learned a lot more about the Bible! Somewhere along the way I learned that "practice of sinning" is to make a habit of sin. Verse 4 is also helpful: "Everyone who makes a practice of sinning also practices lawlessness; sin is lawlessness." Lawlessness might be epitomized by the phrase, "I can do whatever I want!" and indicates there is no sorrow over sin. Verse 6 ("keeps on sinning") adds context which helps us understand "practice of sinning." Again, like Piper has said, for those of us who have been born again, the Spirit will wake us up, shock us out of our indifference, or maybe even cause a trusted friend to speak a word to us.
We should not quench the Spirit (1 Thessalonians 5:19), and we should be aware that sin is deceitful. We are lulled into complacency by the deceitfulness of sin (Hebrews 3:13). It is the nature of sin that we naturally rationalize and justify our participation in it. So as Piper observes, we have cause for concern when we lose our vigilance against sin. But this by itself does not mean that we have not been saved.
By "saved", I mean the concept of justification, which is when a believer is born again and declared righteous by God through faith in Jesus' shed blood and his finished work on the cross to atone for our sins. This occurs at a moment in time. But a person's feelings may or may not reflect the truth of the event that has just occurred. Also (this was very important for me): just because a person can't remember the exact time they were justified does not mean they are not saved. C.S. Lewis and many others have had similar experiences.
In contrast to justification, sanctification is a life-long process that doesn't end until we see Jesus face to face. Every believer is on his or her own journey of sanctification. Along this journey we are becoming more holy and we are being conformed to the image of Christ (Romans 8:29). Sometimes (many times!) we fall backward, and there are times we make no progress at all. But each of us is different. We are in different places on this journey and we are moving at different rates. So take heart, and be encouraged. Stay in fellowship with other believers and cultivate close friendships with them. I find cru.org helpful as a guide to intentional spiritual growth, particularly their concept of a coach (mentor) who is someone farther along in the journey of sanctification: https://www.cru.org/us/en/train-and-grow/spiritual-growth/5-things-you-need-to-live-the-life-god-intended.html
One thing I have discovered while struggling with assurance of salvation: It's not all about knowledge and "figuring out" whether I have been saved or not. It's about relationship. In dry seasons of prayer, I start to doubt my salvation. When I neglect regular Bible reading, I start to doubt my salvation. The feeling is like the anxiety I feel when I have become estranged from a close friend. If Jesus and I are really close, then why don't we talk more?
The burden of intentionally working to build relationship also goes for relationship with fellow believers. It seems as if it has never been so easy to go to church and remain anonymous, but we must resist the temptation to be spiritual consumers who neglect the hard work of relationship-building.
Finally, remember John 6:37: "All that the Father gives me will come to me, and whoever comes to me I will never cast out." Be encouraged!