Am I sinful for my past practices that led up to fornication, even though I have repented from them now, and I was praying during that period?

The abominable acts committed by the questioner are among the greatest major sins and gravest transgressions, and are more severe than the preliminaries to adultery. He must offer sincere repentance to Allah, regret his heinous deed, adhere to Allah's law, and perform abundant good deeds, obligatory acts, and supererogatory acts, for indeed, good deeds efface evil ones. If his repentance is sincere, Allah will accept it and forgive his sins, as Allah Almighty says: ﴿Say, "O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful."﴾ [Quran 39:53], and as the Prophet, peace and blessings be upon him, said: "The one who repents from sin is like one who has no sin."