If our salvation depended on our own good works and whether or not we had been good enough, then you would be right—we never could be sure if we were going to heaven. After all, how could you know whether or not you had been good enough? You couldn't.
But listen: You and I will never be good enough! The reason is because God's standard is nothing less than perfection—and you and I will never be perfect. You see, many people think God must have a set of scales or balances in His hand, and He puts our good deeds on one side of the balance and our bad deeds on the other. If our good deeds outweigh our bad deeds, they think, then He will let us into heaven.
But this isn't true. God is pure and holy, and even one sin would be enough to keep you out of heaven—just one. And that is why we need Christ. He came for one reason: to become the perfect and final sacrifice for our sins. When He died on the cross, all our sins were placed on Him. He took upon Himself the sin and punishment we deserve, and He died in our place.
Can we know we are saved? Yes—when we trust Christ alone for our salvation. Ask Christ to come into your life today, and trust Him—and not yourself—for your salvation. Then believe God's promise to you, for it is true: "Believe in the Lord Jesus, and you will be saved" (Acts 16:31).