There may be some hidden reasons for your fears (such as a childhood experience with illness or death that frightened you). But no matter what the reason is, let me assure you that God loves you, and He wants to take away your fears and give you a new trust in His promise of eternal life.
How can this happen? The key is to let God's promises overcome your doubts and fears. Down inside, your emotions are constantly battering you with messages of fear and anxiety. But listen: If you have truly given your life to Jesus Christ, then those messages are lies! And how do you counteract a lie? You counteract it with the truth the truth of God's Word.
What are the truths you need to bring to your mind? One is that since you have given your life to Jesus Christ, you belong to Him forever. He has made you part of His family, and you are now His child, no matter what happens to you! The Bible says, "How great is the love the Father has lavished on us, that we should be called children of God! And that is what we are!" (1 John 3:1).
Then thank God every day that heaven is real, and it is the inheritance of every person who truly trusts Christ. When doubts or fears arise, turn immediately to God in prayer and ask Him to remind you of these truths. In time your fears will fade in the light of God's unchanging truth.