Will my fasting, prayers, and repentance be accepted after committing major sins, or will that not benefit me?

I praise Allah for guiding you to repentance, which is a great blessing that necessitates gratitude and diligence in obedience. What you heard about the repentance of a major sinner not being accepted is false. Many proofs from the Quran and Sunnah (definition) confirm that Allah accepts repentance from all sins, no matter how great, and that it is a cause for the forgiveness of sins, and even for replacing bad deeds with good ones, as stated in His saying, the Almighty: (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.") And as other verses and hadiths have clarified, be diligent in worship and regret what has passed. Allah rejoices in the repentance of His servant and replaces his bad deeds with good ones.