All Susan ever wanted was to live a normal life. But that plan went totally out the window after she met Charles Kane.
At first, she finds him totally charming and loving. She also has no clue who he is and doesn't realize he's married when they first meet. But after she realizes they've had an affair that has destroyed his political career, she still marries him after his first divorce, which suggests that she loves him enough to feel very loyal to him. On top of that, she falls for him without having any clue that he's rich. She just likes Kane for who he is, and in this sense she represents his best shot at being truly loved in this movie… apart from his buddy Leland, that is.
Here's the really sad thing about Susan's relationship with Kane: she totally gives him everything he wants by loving him for who he is. But he still feels like he needs to do something that'll force her to keep loving him, so he devotes himself to turning her into a famous singer so that he can prove to her and the whole world that he can do anything. Susan goes along with it at first because she loves him so much, but as time goes by, she gets sick of Charles' attempt to turn her into a famous singer.
For Susan, money is "just money. It doesn't mean anything. You never really gave me anything that you care about." In other words, she can see through Kane's façade and study the man underneath. She knows that, deep down, Kane is just a selfish child who wants everyone to behave exactly the way he wants them to. She sums this up by saying to him,
SUSAN: You don't love me. You want me to love you.
In other words, she knows that Kane wants love without having to give any back in return. But as we find out earlier in the movie, Susan is still devastated by Kane's death, as we hear Bernstein tell Thompson,
BERNSTEIN: I called her myself the day after he died […] She couldn't even come to the phone.
Even after she's left Kane and lost all her money, Susan says she isn't so sad about how things have turned out for her. Thompson expresses sympathy for her tough times, but Susan quickly answers,
SUSAN: They haven't been tough on me. I just lost all my money.
In her own words, Susan never wanted money or fame. She wanted love and friends, but these are the things Charles Kane was never able to give her. Even when he offers to give her anything she wanted, he makes the mistake of saying,
KANE: You can't do this to me.
Susan picks up on this immediately and says,
SUSAN: It's you that this is being done to. It's not me at all. Not what it means to me. I can't do this to you? Oh, yes, I can.
So she leaves him and that's that. Susan might have confidence issues at times, but she's not going to spend the rest of her days as Charles Kane's emotional doormat.
P.S. Since the history of this movie is storied and wrapped up in William Randolph Hearst's protests of it, it's worth mentioning that Susan Alexander Kane bears a strong resemblance to Marion Davies, Hearst's mistress—a film actress with a knack for light comedy but whom Hearst saddled with a series of overwrought historical dramas.