Stress is a byproduct of subconscious beliefs you have about the world. You can't choose not to believe something. You believe it because you think it's true. To eliminate stress, you must learn to challenge these beliefs so that you see them differently.
The less you think counterfactually, the less you experience stress. Stress, in this light, isn't a bad thing. It's simply a warning system telling you that your mind has lost touch with what's real.
The reason humans experience so much more stress than other species isn't just because we think more, but also because we think differently.
You might think that shifting your thoughts is as easy as setting your mind to it. But stressful thoughts aren't held in place through choice or will power. They're held in place through perceived truth value.