I know of no Christian who has not experienced doubt about their salvation at least once in their lives. I was saved as a child of about 5 but went through many years of lacking assurance of my salvation because I was shy and didn't want to tell anyone. Then years later, I heard a preacher say that assurance of our salvation is based on God's Word and not our feelings. I had a close friend who was a pastor who said some days he would be so depressed he felt like he was going to hell, and yet he knew he was saved based on the word of God. Take a scripture like John 3:16. It says whoever believes in Christ has eternal life. As a young doubting boy, I looked that up in the Amplified version and saw that the word believes means to "trust in or rely upon". I believed that Christ was indeed who he said he was and that he died for my sins and decided I would rely upon this verse instead of my feelings. Feelings can change rapidly, so our assurance of salvation is not based on them. Doubts of our salvation can come from depending on our feelings, mental depression due to circumstances, sin in our lives, or attacks from Satan. If you believe that Jesus Christ is God and that he died for your sins and you are trusting him to get you to Heaven, rest assured you are saved.
H.A. Ironside, a great preacher from days gone by told of a story where he gave an invitation at the end of the service. A young boy came up and said he just "couldn't see it". He lacked assurance that he was saved. Ironside took his Bible and read the following verses:
"Whoever believes in him is not condemned, but whoever does not believe stands condemned already because he has not believed in the name of God's one and only Son." (John 3:18). Ironside then asked the boy, "What two classes of people do you see here?" The boy answered, "Those who believe and those who do not believe." Ironside then asked him, "Which class are you in?". The boy replied, "Those who believe." Ironside then asked him, "What does it say about those who believe?" It was like a light bulb turned on inside the boy's head. He said, "I see it now. I am not condemned !".
It is like my Uncle once told me about what you believe about Christ. Christ said he was God in the flesh and he would die for our sins. You either have to believe this or you have to believe Jesus Christ was the biggest liar who ever walked the face of this earth. Which do you believe? If you believe Christ is the Savior then you are saved. You have God's written assurance in the Bible.
There are a lot more ways to look at this to give you assurance. Try taking God at His Word and don't worry about your feelings. After a while your feelings will fall in line with your beliefs. If you are still having doubts, shot me an email. I can give you a lot of other information on this subject such as how other people have dealt with it. The main thing to keep in mind is to study and learn more about what salvation really is. I plan to write some articles later to discuss more fully what we get with salvation. It is more than just an escape from Hell. There are a lot of other benefits we come into possession of when we accept Christ. By the way, if you want to send me a private email about your salvation experience and how the doubt arose, please feel free to do so. I might be able to give you a little more specific direction to smash these doubts.
If you are doubting because you feel you cannot live the Christian life, the fact is that no one can apart from the indwelling Holy Spirit. If you try living it on your own, you'll fall flat on your face. If you read the Sermon on the Mount in Matthew 3-6 you will find no one can live up to those standards humanly speaking. The reason Christ gave that sermon is to show people that without his righteousness, no one can hope to please God. Before a person can be saved, they have to realize they can't please God and they are totally bankrupt as far as being able to recommend themselves to God. Then they can see their need of a Savior. When we trust Christ, he gives us (clothes us in) his own righteousness so we stand before God the Father just at perfect and righteous as Jesus Christ himself.