One can have doubts, but they must be accompanied by hope. We are to hope in the mercy of God and in His promises. To assume that we are definitely going to heaven is to commit the sin of “presumption”. Until the moment after death, none of us truly knows for sure where we will spend eternity. We place our trust in God that we are found worthy of His promise of heaven, but Jesus never made a guarantee to us that we would be saved, not even to His disciples. Heaven is not a place for “good people”. Not all “good people” will go to heaven, because one can be good and still be a staunch atheist. You have to seek to know, love and serve Jesus here on earth in order to merit even the possibility of going to heaven. That is why God judges us; He sees us as we are, from the moment of our conception. When doubts overcome you, the best thing to do is pray and tell God, like a child would to a parent, all about it.