Consider Romans.
Specifically, Romans 3:21-26.
Aye, of course it's logically possible God simply won't forgive you. But the basic fact is that logic doesn't hold much with God, and we all know He will.
Here and now, what you did in the past is not so much a threat to your salvation as your doubt is.
You see, nothing man does or has ever done can ever be enough. The works of man are useless to earn our own salvation.
That passage you read speaks of a new justification; not of law, of works, but that of grace, through faith. You can read further through Romans 4 on how Abraham, was counted righteous for faith, not works, and how we, likewise, are considered his spiritual children because we face the same deal.
Romans 5 is about how sin simply can't compete with grace. That's why your past deeds are null and void once you enter this new covenant.
Romans 6,7,8... you may as well read them all.
We all know God is sovereign and nobody tells Him what to do. But we all also know that He is trustworthy, the Most Trustworthy of all.
Yet even more importantly, He's your dad.
You see, we've all been adopted.
(note; adoption: huiothesia, -pronounced hew-tea-seah?- means "entering into a new family relationship with all its privileges and responsibilities").
When we're thinking of adoption, we're not talking about hiring employees. We're talking about full-fledged kinship. And this will not include the things you did before adoption. It's a full step, leave all behind, stand before God 'born again' status. Once you're adopted, nothing in the past counts.
Trust Him on this. Your Dad has a place for you at home. 

I agree with what was said in previous response. Satan doesnt want to let go of us and where there is a weak spot, satan will work onn that spot to try to destroy us and our faith. Be strong and just KNOW that God has forgiven you. If there is anyone that you have not forgiven for something that they have done to you, forgive them. (Matthew 6:14&15) God is faithful and just and will forgive us our sins. God bless 

Ya know, I too find it easy to sin and ask forgiveness and bazillion times and still feel horrible a guilty. It stinks. But, what I've been missing might help you too. If we truly confess and ask for forgiveness (like it's our true desire), then God grants us that forgiveness. The hard part is moving on. See, Satan is the master of deception and guilt. He's gonna pour guilt onto you to make you stay bummed out. Take this for example: A Criminal is on death row. It's plain to see how bad he is by the graffiti he's scribbled on the wall. However, by a pardon from the governor he's set free. The jail door and unlocked and open. But before the prisoner gets up to leave the warden walks and and sits beside him. The warden say, "YOu can't leave. You dont' deserve it. YOu know what you've done. YOu can't really be forgiven for this stuff. YOu're hopeless."--In that same way, Satan tries to trick us and make us think that we cannot move on. God opens the door to freedom. Satan does NOT have the power to close that door, but he does have the ability to try to talk us into not taking advantage of that freedom. Our relationship with God is not always about emotions. That's hard to grasp I know. So when youo've truly sought forgiveness and confessed your sin to God, taste the freedom. Stand up and move on. God will bless you for that, and Satan wil be sorry he ever tempted you in the first place. Hope this info helps (I got lots of this info from a book called Praying God's Word). "Where the Spirit of the Lord is, there is freedom!!"