Praise be to Allah.
We cannot conceal our joy and pleasure at the intention expressed in your question of embracing the true religion, Islam. The confusion and hesitation that you describe are quite understandable, because when a person has been indulging in forbidden relationships and then wishes to move to a religion of purity and chastity, it is natural to feel that his own nafs (desires) will defeat him and that he will not be able to do what Islam requires of him in the way of purity and chastity. But we will tell you some things here which we hope will help you to overcome any difficulty that you may be fearing and give a true picture of the situation.
When a person follows the true religion, it is expected to have a far-reaching effect on his personality and behaviour, so that it re-forms him and gives him a fresh start and a new direction in life, completely different from the way he was during his jaahiliyyah (days of ignorance before his Islam). This radical change will produce morals and values that were not there before, which will purify his heart and fill him with a sense of chastity, so that the new Muslim looks with revulsion and disgust both at his own former deeds and at the ignorant world of promiscuity, infidelity, nakedness and utter corruption that exists in the society around him. He will come back to the sound nature (fitrah the natural state of man) and purity of heart of which Shaytaan had robbed him during the days when he was a non-Muslim and sinner. This new direction in life is undertaken freely, by choice, and is accompanied by contentment and acceptance stemming from his total submission to the commands and prohibitions of Allaah, Who revealed this religion of Islam with all its laws. We have two sources of proof to support what we say here, one from the shareeah (Islamic law)and one from history.
The shari(textual) evidence is to be found in numerous places in the Quraan, for example (interpretation of the meanings):
Is he who was dead (without Faith by ignorance and by disbelief) and We gave him life (by knowledge and by faith) and set for him a light (of Belief) whereby he can walk amongst men, like him who is in the darkness (of disbelief, polytheism and hypocrisy) from which he can never come out? [al-Anaam 6:122]
And those who invoke not any other god along with Allaah, nor kill such life as Allah has forbidden, except for just cause, nor commit illegal sexual intercourse and whoever does this shall receive the punishment.
The torment will be doubled to him on the Day of Resurrection, and he will abide therein in disgrace,
Except those who repent and believe, and do righteous deeds, for those, Allaah will change their sins into good deeds, and Allaah is Oft-Forgiving, Most Merciful. [al-Furqaan 25:68-70]
Concerning the phrase Allaah will change their sins into good deeds the mufassireen (commentators) said: their evil deeds will be replaced by good deeds. Ali ibn Abi Talhah reported that Ibn Abbaas said: These are the believers, who before they believed did evil deeds, but Allaah wanted them to do something else, so He changed their bad deeds into good deeds
Ataa ibn Abi Rabaah said: This is in this world: a man may be doing evil but then Allaah replaces it with good. Saeed ibn Jubayr said: Allaah changed them from worshippers of idols into worshippers of the Most Merciful, and changed them from those who fight the Muslims into those who fight the (polytheists), and from those who marry mushrik (polytheist)women into those who marry believing women. Al-Hasan al-Basri said: Allaah changed them from evil deeds to good deeds, from shirk (polytheism) to ikhlaas (purity of faith), from promiscuity to chastity, from disbelief to Islam. This is the view of Abu Aaliyah, Qutaadah and a group of others.
(Ibn Katheer, Tafseer al-Quraan al-Azeem).
The historical evidence includes a number of stories of Muslims who entered Islam after having been (disbelievers), and how they changed and became righteous. Among these stories is the following:
There was a (Muslim) man called Marthad ibn Abi Marthad who used to smuggle Muslim prisoners-of-war from Makkah (which was the territory of the mushrikeen) to Madeenah (which was the territory of the Muslims). There was also a woman in Makkah, called Anaaq, who was a prostitute. She had been a friend of Marthads (before he became Muslim). Marthad had promised to take one of the prisoners from Makkah to Madeenah. He said: I came to the shade of one of the gardens of Makkah on a moonlit night, then Anaaq came and saw my shadow by the garden. When she reached me, she recognized me and said: Marthad? I said, Marthad. She said: Welcome! Stay with us tonight. I said, O Anaaq, Allaah has forbidden zinaa (unlawful sexual relations). She called out, O people of the camp! This man is stealing away your prisoners! (i.e., she wanted to take revenge on him because he refused to have unlawful relations with her, so she called the polytheists to come and seize him). Eight men pursued me (and he described how Allaah saved him from them). This event was the reason for the revelation of the verse (interpretation of the meaning): Let no man guilty of adultery or fornication marry any but a woman similarly guilty, or an Unbeliever; nor let any but such a man or an Unbeliever marry such a woman; to the Believers such a thing is forbidden. [al-Noor 24:3 Yusuf Alis translation]
(Reported and classed as hasan by al-Tirmidhi, 3101).
The reason for telling this story is that it illustrates how this man changed after he embraced Islam, and refused to commit the haraam deed with which he was tempted. Another story describes a woman who embraced Islam and became righteous:
Abd-Allaah ibn Maghfal reported that there was a woman who had been a prostitute during the days of ignorance (before Islam). A man passed by her, or she passed by him, and he touched her. She said: Stop it! (Mah! A word connoting a rebuke or denunciation). Allaah has done away with polytheism and had brought Islam. So he left her alone and went away.
(Reported by al-Haakim, who said this hadeeth is saheeh according to the conditions of Muslim, although they did not report it).
If you enter Islam and become a truly practicsing Muslim, adhering to this pure shareeah, worshipping Allaah(God willing) as He wants to be worshipped, obeying His commands and heeding His prohibitions, then in sha Allaah you will not encounter or suffer the difficulties you refer to in your question. Moreover, you will have the means of keeping chaste, such as marriage, which will help you to refrain from doing unlawful deeds; marriage is enjoined by this shareeah. The one who chooses a pure and clean way has no need to enter the mire (of illicit relationships). We ask Allaah to guide you and make things easy for you, and to keep evil away from you. May Allaah bless our Prophet Muhammad.