Why do I feel that God is not merciful and forbearing with sinners as the Prophet (peace be upon him) was, which affects my love for God and makes me feel nothing but fear?

The love of Allah is a branch of knowing Him. Therefore, the questioner must first come to know Allah. Allah's mercy is vast, and the mercy shown to creatures is one of the effects of His mercy, exalted be He. Examples have been given of His mercy, glorified be He, which encompasses everything, and His forgiveness of all sins, and His overlooking of missteps, and His appreciation for little work, and His pardon for evil deeds, and His multiplying of good deeds. The best way to know and love Allah is to ponder the Holy Quran.