How do I stop my thoughts from dragging me into fake scenarios of fear and tension?

I really appreciate your question as it is a very interesting one. Regarding the improvement of our attention, it's good to try mindfulness techniques, as you probably are doing by using that meditation that you are talking about. 

While practicing meditation based on mindfulness, it's important to take some tips into account:

1. In the beginning, you should try daily with a little amount of time, 3 minutes is more than enough. 

2. It's completely normal that your mind goes away to your thoughts (memories, your to do list, your worries, judgements ... anything). When this happens, you mustn't blame yourself or feel disappointed, it's completely normal and is the reason why we do it. In fact, to notice that and bring your mind back to the present moment is actually the exercise, so treat yourself kindly at this point. 

3. Try to practice it without too many stimuli around at the beginning, as it's more challenging and is better to try first on your own without distractions. 

4. Try to utilize something to help you put your attention on something, like focusing on your breathing or on the things you see in your room. 

Besides, be aware that meditating is not the only way to develop your span of attention, you can also do other mindfulness techniques:

1. A very good one is putting your attention on the people you speak with during the day. It's also meaningful as you become a good listener. You only need to pay attention to people and whenever your mind goes away, just notice that and bring your mind back to the conversation. Remember to do it without interrupting and being kind to yourself. 

2. You can do some stops during the day to be aware of the present moment. 

3. You can identify your emotions whenever you are feeling them and try to explore and experience them. Whenever your mind goes to your thoughts, bring it back to the physiological experience of your emotions. 

Finally, be aware that, nowadays, there are so many things that can make you be less in the present moment and are not good if you are trying to increase your attention, like your mobile phone in general and specifically social media with their continuous stimulation. 

I am sure you will achieve your purpose. Wish you the best.