Single-mindedness has to do with the power of focus. Your ability to focus is ultimately your biggest ally in accomplishing tasks and projects. With practice this is an ability one can develop. Your level of achievement and success depends upon it.
Define single-mindedness: It is an adjective meaning: 1) Determined 2) Having one overriding purpose or goal. 3) Steadfast; resolute.
Now let's define focus. 1) The concentration of attention or energy on something. The opposite of this would be "absent-minded", "scattered", or perhaps just "distracted."
Positive and Negative Polarity
Know that there are positive forms of "single-mindedness" and negative forms. The negative influence of single mindedness: When one becomes so preoccupied with a task that they lose all grip with their daily responsibilities and Accountability to others, this is obviously a form of self delusion. These personality types can be self-centered or self obsessed people (STS), oblivious to people and things impacted around them. Often people that are mentally ill or mentally challenged will be seen doing the same movements over and over. This is not the higher quality focus of Single-mindedness that we develop to help improve our Concentration.
The positive form of Single-mindedness is the ability to block out distractions and erroneous or negative thoughts in order to concentrate better. Your desire to become more single-minded will enable you to finish projects with more efficiency in less time.
Developing Power of Focus
Practice the common methods and some not so common methods to develop your power of focus:
Creating the environment is step 1. If it's a task where you are seated at a desk then you will need to clean the desk area and become more organized. At that time a new project starts with a clean slate. Let's suppose you are trying to quit smoking. Create the environment to allow this to be possible. In other words on your quit date the house and car you occupy need to be smokeless in every way. No ashtrays, lighters, etc.
The power of Intention. This is a slightly different perspective than mere goal setting. Intention means on the deepest level figure out exactly what is you intend on creating, then affirming this to yourself and visualizing an approximate end result, i.e. being a non-smoker, finishing a work assignment, writing a college thesis, etc.
Set and setting. In other words where are you most likely be able to focus or accomplish your goal? With what external conditions around you are tolerable to your concentration abilities? For example if your intention is to be a non-smoker then you are more likely to succeed hanging out in a local gym, vs. the local bar. If you are trying to finish studying for college exams, you may want to create privacy vs. the kids running around and playing in your study space.
Set small steps and short term goals. Do this in writing in outline form. Breaking down the steps can help the mind from not getting overwhelmed.
Now Moment, Mindfulness
The Controllers have zero power over you (one's thoughts and emotional state) when you are outside of their frequency reach by maintaining neutral balance or the Observer Point. Being in the Observer role also means that you are fully present in the Now Moment, which can also be referred to as Mindfulness.
Mindfulness is the psychological process of bringing one's attention to experiences occurring in the present moment, which one can develop through the practice of Meditation and through negative ego clearing and mental discipline. Mindfulness is derived from sati, a significant element of Buddhist traditions, and based on Zen, Vipassanā, and Tibetan meditation techniques. 
- [Singleminded Wikihow]