Positive thinking has been the dominant self-help philosophy since Norman Vincent Peale wrote a book about it in 1952. The basic idea is that if you think positive thoughts, positive things will happen. This has been thoroughly debunked by science, and also by anyone who has ever thought "I hope my flight isn't delayed" in an airport.
Pessimists are not surprised by bad outcomes. They've already imagined every possible way things could go wrong, so when things go wrong, they're prepared. The optimist shows up at the airport expecting an on-time departure and is devastated by a two-hour delay. The pessimist packed snacks, charged their devices, and brought a book. The pessimist is fine. The pessimist is always fine, because the pessimist never expected things to go well in the first place.
There's a real psychological strategy called defensive pessimism. You imagine the worst-case scenario, figure out how to deal with it, and then proceed. It lowers anxiety because you've already faced the fear. It improves performance because you've planned for problems. It's everything positive thinking claims to be, except it actually works and doesn't require you to lie to yourself about your vibrational frequency.
Toxic positivity is when someone tells you to "look on the bright side" while your life is objectively difficult. It dismisses real problems. It makes people feel guilty for having normal human reactions to bad situations. "Everything happens for a reason" is not comforting when the reason is "the universe is indifferent and sometimes bad things just happen." Negative thinking gives you permission to acknowledge reality, which is the first step toward actually dealing with it.
Expect things to be hard. Plan for them to go wrong. Be pleasantly surprised when they don't. Lower your expectations to a manageable level and then occasionally exceed them. This is not depression. This is risk management with better branding.