Why didn't we feel our existence in the past, and why do we only begin to have consciousness and perception after birth?

It appears that the questioner is asking about the reality of humanity, encompassing both body and soul. This reality was non-existent before its creation, as affirmed by Allah's statement to His Prophet Zechariah: "And indeed I created you before, when you were nothing" (Maryam: 9). That which is non-existent is not a thing.

As for why consciousness and perception do not begin until after birth, this is part of Allah's wisdom and mercy. Ibn al-Qayyim mentioned that children, while in the womb, are stronger than they are after their birth.

Some have cited the verse: "And Allah has extracted you from the wombs of your mothers while you knew nothing, and He made for you hearing and vision and intellect that perhaps you would be grateful" (An-Nahl: 78) as proof that humans are given hearing and sight after birth. However, Ibn al-Qayyim refuted this, stating that the verse is actually an argument against them, because the heart (intellect) is created in the mother's womb. Furthermore, the hadith of Hudhayfah ibn Usayd in the Sahih confirms that hearing and sight are created for the fetus after forty-two nights. Thus, the faculties of hearing and sight are placed within it, and actual perception is contingent upon the removal of the veil that prevents it, by exiting the womb.