I remember one of the exercises that were described by the doctor of psychological sciences Yu. M. Orlov. You need to take some object and look at it for about 10-15 minutes, thinking only about this object, not being distracted by anything. So you can do your job efficiently and not make mistakes due to carelessness.
You have uncontrollable jumping of thoughts from an arbitrary topic to "your thoughts". This neurotic behavior (and this is not a name-calling and not a diagnosis for your life, and not even a disadvantage), which can be changed not by training mindfulness, but by the attention of another person (the psychotherapist first of all) to your "thoughts". You come back to them all the time, and there is something at the same time important and intolerable, unappropriated experience. What specifically needs to be studied together, now I can only provoke this journey a little with my imagination.
I would have such intermittent attention if someone indifferently annoyed readily followed my intellectual affairs catch me on vacation. If he noticed my distant gaze and relaxed posture, he would perceive them as a protest against my "homework" and would start criticizing me for my lack of diligence and responsibility, maybe even yelling at me ... then over time I would learn to catch moments myself distraction and would immediately strain in anticipation of criticism and anger.
Perhaps the reason is in the mistakes of your parents in your infancy.
One of the most obvious ways is solitaire games, games like Lines and the like. A more classic version is a "spy" game with moving objects on the table (remember "The Secret of Two Oceans"?)
In addition, there is a whole arsenal of exercises, at first glance, not directly related to attention. For example, our children's training "Kuklachev is resting!" - krivolapchuk.ru