Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Dear brother, we would like to thank you for the great confidence you place in us, and we implore Allah Almighty to help us serve His cause and render our work for His Sake.
In his response to your question, Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states,
In Islam, we earnestly believe that Allah is All-Compassionate and All-Forgiving and that He is ever willing to forgive those of His servants who turn sincerely to Him in repentance and ask forgiveness of Him. Allah says, “Verily, I am Forgiving to him who repents and believes (in Me) and does good works, then follows the right path.” (Taha 60:82)
Sincere repentance, however, is not mere words. Rather it involves tangible steps that one must take that reflects true remorse. These steps include feeling true sorrow and remorse for one’s sin; refraining from the sin as well as all the circumstances that lead to it; resolving never to repeat the same again; and finally, following it up with whatever good deeds one can afford to do.
The Prophet (peace and blessings be upon him) said, “If you happen to commit a bad deed, follow it up with good deeds in order to wipe it out.” (At-Tirmidhi)
Once you have taken the above measures, you can certainly hope to receive the mercy and forgiveness of Allah as He Himself has promised. The Prophet (peace and blessings be upon him) said, “A person who has sincerely repented of a sin is like one who has never sinned!” (Ibn Majah)
Having changed yourself for a better person, there is nothing wrong with you marrying this woman, but it is still a good idea for you to tell her that you were not a good Muslim in the past so that she makes up her own mind.
While doing so, you must never divulge the specific sins you have committed that Allah has covered for you, for by doing so you are removing the cover that the Merciful Lord has placed over them. An exception to this, however, must be made: If because of your fling in the past you have contracted any contagious [sexually transmitted] disease or possibility of contracting it, then you must divulge the same in order to protect your future partner.
Let me conclude: Never let past sins weigh you down. Look to the future and the infinite Mercy of Allah, who is All-Compassionate and All-Merciful. Since Islam is above all a religion of mercy, Islamic history is full of sinners turned into saints.
So, look forward to the Mercy of Allah which He has promised to those who flee to Him in repentance and good works.
Allah Almighty knows best.
Excerpted, with slight modifications, from: www.muslims.ca