It won’t help to simply scold yourself for irrational thinking. Cognitive-behavioral therapy requires that you identify the specific irrational thoughts and challenge them directly. For example:
You read one of her texts and you think: She’s having doubts about whether I’m the right guy for her. She’s thinking of breaking up with me.
Albert Ellis or a Cog-B therapist would take that thought and re-process it this way:
What is the evidence for this? Is there another way of looking at what she said? What is the contrary evidence? If it did turn out to be true, would it really be catastrophic?
It’s also worthwhile to enhance your self-awareness of your own self-worth—why any woman would be foolish to break up with you.
Rather than berate yourself, sympathize with yourself for having fears of losing her: Of course you don’t want the pain that would accompany that. Nobody would. Try to accept your fears. Then reinforce your knowledge that you have the inner strength to survive that loss if it should happen.
Off topic, but if you think the characters in American Psycho are just "successful people looking after themselves" I think you missed the point of the movie. I've never seen a more perfect portrayal of Peter Keatings.