Where does pride stem from? And why am I so prideful lately?

Pride [superbia] is so called because a man thereby aims higher [supra] than he is; wherefore Isidore says (Etym. x): "A man is said to be proud, because he wishes to appear above (super) what he really is"; for he who wishes to overstep beyond what he is, is proud. Now right reason requires that every man's will should tend to that which is proportionate to him. Therefore it is evident that pride denotes something opposed to right reason, and this shows it to have the character of sin, because according to Dionysius (Div. Nom. iv, 4), "the soul's evil is to be opposed to reason."

Pride is not the same as vainglory, but is the cause thereof: for pride covets excellence inordinately: while vainglory covets the outward show of excellence.

Some pride you are born with,some is learned.
We are raised to set our goal as being the best.

This is inspiring that you want to change. Humility says I need to change. This is the start. There is good news. God wants to meet you here. When people humble themselves He desires to reach out and help. He supports you so much in your desire for change. So cry out to Him!