First, don't worry about hell. Don't let fear of hell rule you - in fact, forget hell even exits. It's not worth thinking of. Instead, let your desire to serve God rule you.
Second, if you know something is wrong in God's eyes, why would you think it's okay? Why would you allow yourself to purposefully defy God? He will forgive, but not just because you ask Him to. He will forgive if you repent, promise never to commit that sin again, and atone. You must understand why you did wrong and seek to amend it. If you commit a sin thinking, "God will forgive me for this later," odds are you won't be worthy of His forgiveness. Christianity is not a reward-punishment system.
Do you understand why sex is wrong before marriage? Use of contraception is sinful because it prevents God from creating new life when you do the act that is meant to create new life. It is mocking God, and it defiles your bodies. It is a highly selfish act that places you before God, which is always wrong. If you have unprotected sex outside of marriage, you risk having a child and bringing that child into an unstable unmarried family. You would do that child great harm because you're not able to support and care for the child since you're not even able to support and care for yourself.
Also realize that when you have sex you give a huge part of your soul to that person. You will NEVER get that part of your soul back. Wouldn't you be happy to have your spouse keep that part of your soul? What if it was some person you hate that you slept with 10 years ago that stole that part of your soul? You'd always feel incomplete and resentful.
Married Roman Catholic