That's not true.
You can know that you are going to heaven without a shadow of a doubt. Not because of what you are or what you've done. Those things won't get you to heaven. But because of what Jesus did for you by dying in your place on an old rugged cross two thousand years ago; because you have believed it and received it and because you are trusting in His unshakeable promises. Look at the Scriptures, like John 3:36; 5:24; 10:27, 28; 14:1-3; Rom 10:9; 2 Tim 1:12; 1 John 2:3 - 5, and be encouraged.
I have a growing conviction that there two things that we ought to be doing in these days. The first is fulfilling the Great Commission: going into all the world and preaching the good news of the gospel.
The second is living in the light of eternity. It seems to me that there are a lot of Christians who know that their sins are forgiven, know that they are going to heaven, know that their future is secure, but who are living entirely for this life, with little or no thought of where they are going to be or what they are going to be doing in eternity. This life is short; eternity is a long time. This life is preparation for what's to come.
My friend Denny Hartford is an American, a Christian and a pro-life activist. He writes a regular blog. He gets more done in a day than I do in quite a bit longer than that (but then he's a few years younger than I am). Among other things, he helps out as a teaching pastor at a church in the city where he lives.
On Sundays, the church normally meets just once, as I understand. But recently he invited the congregation to join him on Sunday evenings in a series of studies based on a remarkable book called Heaven by an excellent Christian author named Randy Alcorn. What a wonderful idea for a church!
If there isn't anything like that at your church, why not start a similar study at home with your own family? I don't know if they will have the book at your local Christian bookstore, but I notice they have it at Amazon.
Some Christians believe that when they die, they are going to heaven for ever. Other Christians point out that there's going to be a new earth, and suggest, on the basis of Rev 21:1 - 3, that God's ultimate purpose is not to get people to heaven, but to get them to the point where He can come down to earth and dwell in the midst of His people. And where, they might ask, are you going to be when Jesus comes to earth a second time, as He promised (1 Thess 4:17; Jude 14)?
But if we talk about those things, it's going to have to be another time. . .