The Source(s) of our Character The Yetzer haRa and the Yetzer haTov
1 א

(ז) רבי נחמן בר שמואל בר נחמן בשם רב שמואל בר נחמן אמר: הנה טוב מאד, זה יצר טוב. והנה טוב מאד, זה יצר רע.

God created both good and evil

citing from Genesis, the Midrash expounds:

'Behold, it was good' refers to the Good Desire (Yetzer haTov)

'And behold, it was very good' refers to the Evil Desire. (Yetzer haRa)

2 ב

And these desires continue to be re-created at Birth:

The yetzer hara is 13 years older than the yetzer hatov: While still in the mother's womb, the yetzer hara begins to develop in a person. If he begins to violate the Sabbath, nothing stops him. If he commits murder, nothing stops him. If he goes off to another sin, nothing stops him.

But 13 years later, the yetzer hatov is born. When he violates the Sabbath, it rebukes him, "Airhead [literally: "empty one"]! Don't you know it says 'Everyone who violates it will surely be put to death' (Exodus 31:14)?" If he is about to commit murder, it rebukes him, "Airhead! Don't you know it says 'Whoever sheds a man's blood, by man will his blood be shed' (Genesis 9:6)?" If he is about to engage in a sexual sin, it rebukes him, "Airhead! Don't you know it says 'Both the adulterer and the adulteress will surely be put to death' (Leviticus 20:10)?" (Avot d'Rabbi Natan 16).

3 ג

Nature AND Nurture

Maimonides: Hilchot Deot 1:2

With regard to all character traits: a person has some from the beginning of conception, in accordance with that person's nature... Some traits are not from birth. They may have been learned them from other people... or may have come as a result of one's own thoughts, or because one heard that this was a proper trait.

4 ד

We once had the opportunity to kill off the Evil inclination:

The Sages decided that they were going to capture and imprison the Yetzer HaRa. So they ordered a complete fast of three day….whereupon he [the Yetzer haRa] was surrendered to them...

but he [the Yetzer] said to them, “Realize that if you kill me, the world is finished.” They held him for three days, then they looked in the whole land of Israel and not an egg could be found. So they asked, “What shall we do now?”…So [instead of killing him] they blinded him and let him go; (Talmud Bavli, Yoma 69b)

5ה

טבע האדם תלוי במנהג והמנהג יוקבע בטבע

Given that we didn't kill Evil, how can a person change?

A person's character depends on their habits, and integrating those habits as a part of the person. - Iggerot HaRambam 12:26

6 ו

This is really obvious. Why are you teaching me something I already know?

"I have written this work not to teach people what they do not know, but to remind them of what they already know and is very evident to them. You will find in most of my words only things that most people know and about which they have no doubts. But just as these things are well known and their truths are revealed to all, so too is forgetfulness in relation to them extremely prevalent. It follows, then, and the benefit to be obtained from this work will not be derived from a single reading. - Moshe Chaim Luzzatto in his introduction to Mesilat Yesharim

7 ז

A Cautionary Word about Perfection:

Should we despair of our being unable to attain perfect purity? We should, if perfection were our goal. However, we are not obliged to be perfect once and for all, but only to rise again and again beyond the level of the self. - Abraham Joshua Heschel