Be careful of what you say! How can you doubt having sins greater than God's Mercy? God said, “My anger falls upon My servant who thinks his sin is greater than My Forgiveness.”
Here are some things about God that will make you feel much better (these quotes are all based on the sayings of Prophet Muhammad PBUH):
God said, "I swear by My Glory and Honour, if they ask for forgiveness, I will forgive them. Whosoever repents and comes to Me, I will receive him though he is far away. And whosoever turns his back, I will call him closely telling him, ‘O My servant, where are you going? Have you got any other God besides Me?’ "
God said, "O son of Adam, so long as you call upon Me and ask of Me, I shall forgive you for what you have done, and I shall not mind. O son of Adam, were your sins to reach the clouds of the sky and were you then to ask forgiveness of Me, I would forgive you. O son of Adam, were you to come to Me with sins nearly as great as the earth and were you then to face Me, ascribing no partners to Me, I would bring you forgiveness nearly as great as it. I appreciate even the slightest effort, and I forgive even the biggest sin. My Mercy precedes My wrath, and I am more merciful to My servants than a mother is to her newly born child.”
A man said, “By God, God will not forgive so-and-so. At this, God the Almighty said, “Who is he who swears by Me that I will not forgive so-and-so? Verily I have forgiven so-and-so and have nullified your [own good] deeds!”
The Messenger PBUH said, “When one of God’s servants asks for forgiveness and insists, God says, ‘Truly My Mercy is upon My servant.’ So God calls Gabriel, ‘My Mercy is upon so-and-so!’ Gabriel then calls The Holders of The Throne, ‘God’s Mercy is upon so-and-so!’ The Holders of The Throne then call the inhabitants of the heavens, ‘God’s mercy is upon so-and-so!’ Then the inhabitants of the heavens say, ‘Truly, the Mercy of God is upon so-and-so!’ ”
A man came to Muhammad PBUH and said, “I am a man who has committed great sins. Could God ever forgive me if I repented?” So Muhammad PBUH replied, “Yes, He would forgive you all your sins!” So the man said, “O Prophet of God, and my betrayals and debaucheries?” So Muhammad PBUH answered, “And your betrayals and debaucheries!” The man then repeated, “And my betrayals and debaucheries?” So he said, “And your betrayals and debaucheries!” So the man lifted his arms to the sky and cried out loud, “Allahu-Akbar! He is ‘The Pardoner’ who forgives us our sins!”
Another man came to the Prophet PBUH and asked, “Who will handle our judgement on the Day of Resurrection?” So Muhammad PBUH said, “God will.” The man then asked, “Will God do it personally?” He said “Yes, He will.” At that, the man smiled and rejoiced. So the Prophet PBUH asked him, “Why are you smiling?” The man replied, “The Generous pardons when he can, and when He judges, He forgives.” So Muhammad PBUH said, “You have spoken the truth, for there is no one who is more generous than God.”
In another instance, a man asked Muhammad PBUH: “If I committed a sin, will it be written?” He said, “Yes, the angels will write it in your book of deeds.” The man asked, “What if I repented?” The Prophet PBUH said, “It will be erased.” The man then asked, “What if I returned to it?” The Prophet PBUH said, “It will be written again.” The man continued, “And if I repented?” He answered, “It will be erased again.” The man repeated his question several times and then asked, “How many times will it be erased?” The Prophet PBUH said, “God does not ‘tire’ from forgiving until you tire from repentance.”
The Prophet PBUH said, “God has created a door to the West, as wide as a seventy-year walk. This door was created the very same day the heavens and the earth were created.” So he was asked, “And what is this door, O Prophet of God?” He replied, “This is the ‘Door of Repentance’. It shall never be closed till the sun rises from its direction. ”
“Whenever someone commits a sin and then says, ‘O my Lord! I have sinned, please forgive me!’ God says, ‘My servant has known that he has a Lord who forgives sins and punishes for it. I, therefore, have forgiven my servant.’ If that person commits another sin and says, ‘O my Lord, I have committed another sin, please forgive me,’ God says, ‘My servant has known that he has a Lord who forgives sins and punishes for it. I, therefore, have forgiven my servant.’ Then, if that person commits another sin and says, ‘O my Lord, I have committed another sin, please forgive me,’ God says, ‘My servant has known that he has a Lord who forgives sins and punishes for it. I, therefore, have forgiven My servant, he can do whatever he likes.’ ”
Truly God is the Lord of Mercy, the Giver of Mercy.
If you want, you can check out the book I've written to learn more about Islam. Here's the link:
http://spaces.msn.com/slimayn/
I hope this is not too long.
If you have any questions, plz dont hesitate to contact me :)
Peace be with you
EDIT: Yes a muslim man would be allowed to marry you. If God forgives you - and He will if you repent sincerely - than why would you not be allowed to marry a Muslim? and who is he that can tell you otherwise when God affirms that He forgives all His servants' sins? Remember than the moment one converts to Islam, he/she starts with a clean slate. Past sins do not matter anymore. So there's nothing to worry about