Are there any negatives to positive thinking?

My wife is taking a big exam in a few days. She’s been studying for this exam hours per day and has spent years in school to prepare her for this. She’s nervous, and her mind has been engaged in all kinds of “what if I don’t pass” scenarios, which simply increase her anxiety about the test. She catastrophizes and her mind will live in a fantasy about how bad everything will be if she fails.

There are a few people she knows who believe in “positive thinking” – telling her that she will pass the exam. Their recommendation is that she should tell herself that she will pass the exam, and even picture herself passing it.

This could be entirely useful and legitimate advice. But I am confused by it, as it seems that “what if I fail?” and “I will pass” both live in the same space. They are both bringing her from her present activity of studying.

These two approaches seem to be built upon the same promise – that there is value in spending her studying time now pretending that she is sitting for the exam. It appears to be about hardening expectations, rather than keeping them open.

What if she were to approach the exam without expectation (good or bad)? Wouldn’t this allow her to devote her full attention to doing what she wants to do, which is to study? In my experience, letting go of expectations allows me to focus on what I find important. It also allows me to enter the event/activity with more curiosity and less anxiety.

Note: I’m not even sure how I was going to word this question. It could be that there are different approaches that work better for everyone. And I could be completely wrong. It may be that positive thinking is great and has no real negatives at all. I’m just curious to hear what people have to say about it. If you’re an pessimist and have practiced positive thinking, in what form did it take, and how did it affect you?