Let’s start by a question, what is the thing you have control on in your life? Is it the past, the future, or the present? As obvious the answer can be, what most of us think about is either the past or the future.
The past moments cannot be changed, and are now in the memories. The future cannot be predicted. It is the result of our actions at the present moment. There are too many factors that can change the future, this only makes it hard to control it. The only thing we have control over is the present moment. Whatever and however it is, the present is the only thing you have control over.
The issue with constantly thinking about the past and the future is that the more you do it, the more you start to imagine some bad possible situations. The mind can start to create some possible catastrophes that are far worse than what might actually happen.
To take things one day at a time is to actively focus our thoughts and attention on the current moment. Taking less care about what is going in your mind, and more on what is around you. Engaging in activities that are happening now, where you focus your thoughts on what you actively sense will make it much harder for your mind to think about the past and future.
Most people know how living in the present is important. But the paradox here is that when someone is thinking about the past or the future (aka not living the moment), he is not aware of that. To be able to live the present takes some practice. Especially when thinking about the future or the past is integrated in our life. Breaking the habit can be challenging at first, but constantly taking small steps toward actively living the moment is the key to breaking the loop.
Another way to actually take things one day at a time is to focus more on the process rather than the outcome itself. It would be much easier to focus on just one piece of the puzzle rather than at all pieces at once.