Fear Of Not Being Forgiven
God has clearly mentioned that He would forgive on sincere repentance. The incident you refer to is about the enemies of God, who had committed major crimes and never repented, and at the time of the conquest of Makkah, the Prophet declared that no mercy for them, even if they requested peace in the Kaaba. This has nothing to do with sincere repentance. You should read the Quran: See 39: 53 قُلْ يَا عِبَادِيَ الَّذِينَ أَسْرَفُوا عَلَىٰ أَنفُسِهِمْ لَا تَقْنَطُوا مِن رَّحْمَةِ اللَّهِ ۚ إِنَّ اللَّهَ يَغْفِرُ الذُّنُوبَ جَمِيعًا ۚ إِنَّهُ هُوَ الْغَفُورُ الرَّحِيمُ