God doesn't want you to be filled with doubt about your salvation, or spend the rest of your life wondering if He'll welcome you into heaven when you die.
Instead, He wants you to have confidence in what He has promised us in His Word: That if we've truly trusted Christ for our salvation, God has already given us the gift of eternal life. The Bible says, "I write these things to you who believe in the name of the Son of God so that you may know that you have eternal life" (1 John 5:13). As I heard someone say many years ago, "God said it... I believe it... so that settles it!"
Let me ask you a question: When you gave your life to Jesus as a teenager, what exactly were you doing? Were you just promising to live a better life? No, it was something far greater than that. What you were doing was trusting Christ -- and not yourself -- for your salvation. You realized that Jesus Christ had died for you, and He took away all your sins.
Don't trust your feelings, for they can deceive you (especially when you aren't feeling well). Don't trust in your own goodness, either -- for God's standard is perfection, and no one measures up to that standard. Instead trust Christ and what He did by His death on the cross and His resurrection from the dead for you. He alone is our hope.