I think there are many people who are sinners nowadays, since more and more people do not marry or are no longer virgins before marriage, and many people have sex with several people in their life. There are also more and more children born out of wedlock. Does this mean there are already billions of people in hell and all the sinners go there after their deaths? It is really difficult in our modern society to avoid all these sins which are trivialized. Is confession a way to wash away sins or is it too late once you're a sinner?
I consider myself as a Christian, I believe in God, but I am a sinner. I don't know what to do to be forgiven by God. This girl I spent a night with is really attractive, but I feel guilty.
The number of people who sin doesn't make sin better. After all, just before God destroyed the world in Noah's day, it is said, "Then the LORD saw that the wickedness of man was great in the earth, and that every intent of the thoughts of his heart was only evil continually" (Genesis 6:5). All those sinners didn't make their sins acceptable. Even Jesus warned, "Enter by the narrow gate; for wide is the gate and broad is the way that leads to destruction, and there are many who go in by it. Because narrow is the gate and difficult is the way which leads to life, and there are few who find it" (Matthew 7:13-14). Sadly, you are right, there are a lot of people heading to hell and it is really hard to get people to listen to God.
I know it is common to think today's world is different from the past, but people are people. Sexual sins have always been a problem, which is why they are discussed in the Bible. Paul described the Gentile society in his day. After talking about how they ignored God and created their own gods, he said, "Therefore God also gave them up to uncleanness, in the lusts of their hearts, to dishonor their bodies among themselves" (Romans 1:24). Reading through the Old Testament it isn't hard to find many examples of people committing sexual sins.
God's goal is to save as many people as He can from their sins. "The Lord is not slack concerning His promise, as some count slackness, but is longsuffering toward us, not willing that any should perish but that all should come to repentance" (II Peter 3:9). That is why Jesus came into this world to die for us. We cannot rescue ourselves from our own problems, so Jesus provides us with a way out.
Considering yourself a Christian is a bit different from knowing you actually are a Christian. I would like you to read through: How to Become a Christian and What Must I Do to Be Saved? For a Christian who has sinned, there is a need for repentance (a change) and confession. See: "I feel so guilty about my past sins. I just can't get them off my mind. Why do I still not feel forgiven? Is forgiveness all about feelings?" Confession alone would not be enough. Otherwise, you would have people sinning without restraint thinking all I need to do is confess it and then go right back to what I was doing.
"Attractive" explains why you were distracted and gave into sin, but it never serves as an excuse for sin. Oddly, I'm glad you feel guilty. The people who sin and it doesn't bother them are the most difficult ones to deal with.