All Catra ever wanted was Adora. But, she's under the misguided impression that Adora left her for the Rebellion. So her new objective is to lash out. Lash out at the Rebellion for taking Adora away from her. Lash out at the princesses and basically all of Adora's new friends for thinking that they deserve her more than she does. But more than anything, lash out at Adora herself. Her belief is that if she can't have Adora, no one can. Simply speaking, Catra is hurt, confused, and just really realy messed up. She's actually something like a yandere, but not quite. But much like some famous yanderes, if Catra ever did manage to kill Adora, it would be almost guaranteed that she'd keep something of hers as a memento, if she didn't just kill herself immediately after.
Of course, all of this is happening while at the back of her mind, she realizes she's making a mistake. The episode inside the portal illustrates this more clearly than anything. She wishes she could do things over where everything was back to how it was before Adora defected. And even after she realizes the world is a lie, she pretends she doesn't see it, and asks Adora "why can't you just stay"? In fact, Catra asking Adora to stay with her is incredibly prevalent throughout that episode, in addition to her claiming everything was perfect.
But I've gone on too long about this, and am dangerously on the verge of digression. I hate to quote twitter, but this screenshot sums things up pretty nicely on how it completely encapsulates Catra, not only in the portal but just in general.