On one hand, God has certainly forgiven all sins and you do not have to worry about them. However, I do think there are situations where remarriage is not an option for one who is determined to remain obedient to God. With what you have told me, I have to think that this applies to your situation.
However, I would first like to address one doctrinal issue. You mention the popular idea of living in adultery. Though many people teach this, I do not. Let me explain. I believe that the Bible teaches the sin of committing adultery--not living in adultery. Consider these verses:
Matthew 19:9 And I say unto you, Whosoever shall put away his wife, except it be for fornication, and shall marry another, committeth adultery: and whoso marrieth her which is put away doth commit adultery.
Mark 10:12 And if a woman shall put away her husband, and be married to another, she committeth adultery.
Adultery is the act of breaking the marriage vows. When one is unlawfully divorced and marries another person, that breaks the marriage vow and the sin of adultery is committed. However, God never says that the person is living in adultery. If that were the case, the only way to get right with God would be to leave the second spouse (no matter how long or how many children had been born to the second union) and try to return to the first spouse. But God never asks anyone to do this.
Wrongful remarriage is an act of adultery. It is a sin that needs to be forgiven. However, it is not a state of sin. If it were, all wrongfully remarried people should get divorced when they get saved or right with God. It is sin, but it cannot and should not be reversed. Compare adultery with murder. If a person murders someone, they committed the sin of murder. Yet, although the murdered person cannot be brought back and the sin cannot be reversed, no one thinks of the person who committed the murder as living in murder. He is a murderer because of the past, but he is not living in murder.
Therefore, the problem isn't whether or not your first husband died; it is a matter of whether any of your divorces were against the commandment of God. If they were, you should not remarry.
Remember, God's grace is sufficient. He will help you all your days. Allow this to give you a special opportunity to give yourself to His service. You will find His yoke to be easy and His burden to be light.