Repentance wipes out whatever came before it
One might say: “I want to repent, but who can guarantee that Allaah will forgive me if I do so? I want to follow the Straight Path, but I feel very hesitant. If I knew for sure that Allaah would forgive me, I would definitely repent.”
I would say in response that these feelings of hesitation are the same as those experienced by the Sahaabah themselves. If you think seriously about the following two reports, the feelings you have will be dispelled, in sha Allaah.
Imaam Muslim (may Allaah have mercy on him) reported the story of how ‘Amr ibn al-‘Aas (may Allaah be pleased with him) came to Islam: “…When Allaah put the love of Islam into my heart, I came to the Prophet 
and said: ‘Stretch forth your hand so that I may give bay’ah (pledge of allegiance) to you. He stretched forth his hand, but I pulled my own hand back. He asked, ‘What is the matter, O ‘Amr?’ I said, ‘There is a condition.’ He asked, ‘What is the condition?’ I said: ‘That I be forgiven.’ He said: ‘Do you not know, O ‘Amr, that Islam wipes out whatever came before, hijrah (migration for the sake of Allah) wipes out whatever came before, and Hajj wipes out whatever came before?’”
Imaam Muslim reported from Ibn ‘Abbaas (may Allaah be pleased with them both) that some people among the mushrikeen killed, and killed a great deal, and they committed adultery, and did it a great deal. Then they came to Muhammad and said: “What you say and advocate is good, if only you could tell us that there is any expiation for what we have done.” Then Allaah revealed the words (interpretation of the meanings): “And those who invoke not any other god along with Allaah, nor kill such life as Allaah has forbidden, except for just cause, nor commit illegal sexual intercourse – and whoever does this shall receive the punishment.” [al-Furqaan 25:68]; “Say: ‘O My slaves who have transgressed against themselves (by committing evil deeds and sins)! Despair not of the Mercy of Allaah…” [al-Zumar 39:53
to be continued