How can one remain steadfast in repentance and abandon sins after repeatedly returning to them? And does Allah forgive what has passed of them? And what is the ruling on applying light medical foundation to conceal skin pallor?

The door of repentance is open. You must offer sincere repentance and regret your sins, and commit to righteousness. What will help you in this is the company of righteous women, striving against your lower self, seeking Islamic knowledge, and constantly engaging in remembrance (dhikr) and supplication (du'a). A woman going out with makeup is not permissible, whether it is light or not light.