I’m going to get tough on your heroine here because I think you’ve come up with good material.
Justyna has lived a privileged life. She lies and cheats. She must do it often because she’s good at it. Chances are that it is both a means to an end and a coping mechanism.She apologizes at times, but rationalizes her actions, which makes her apologies suspect. She probably shades the truth and manipulates about smaller things. (Just rebalancing the scales to get what she deserves and to be seen as better than others.)
This sounds like a villain, but, she does not act from malice. She does not intend to hurt other people. She actively works to be kind, even sacrificing at times for others. It’s just hard for her to do so in some circumstances when her well-deserved place in the hierarchy is at risk.
Mostly, this works. She doesn’t notice any of the havoc she creates for others, the honest people she displaces, or the way it weakens her connection with others. Her principles are eroding, but these feel like minor exceptions that are either beyond her control or trumped by the greater good of getting what she wants.
So there is a wonderful story developing here of Justyna making bad choices, worrying readers about future consequences and perhaps a turn to the dark side. Then the lies and cheating don’t work as well. Consequences pinch harder and harder. Losing something greater than status — love — becomes real to her. At the root of all the lying and cheating is pride. And she has to deal with that (probably with a grand gesture) to be worthy of a happily ever after.
This is one way, a valid ways (but probably the wrong way), to view the story in terms of character arc. Note that it is abstract and focused on unhealthy behaviors and a deeper flaw. This is generic stuff, not the story itself.But I hope there’s a tension here even in the bare bones description.