That's a problem sometimes referred to as tunnel vision and something that everyone struggles with at one time or another. Check the forum archives. There are about a dozen previous threads on dealing with tunnel vision. There may be some articles as well; I didn't check. Good luck.
Focus During Chess
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Remember to calculate. Start with plenty of moves that might be interesting, only then you may narrow it down and calculate a little deeper. Now, what is interesting? For that you need to look at the position. There are things like ''weak king position,'' ''pieces hanging in the air'', and ''pieces that are in a clutter or on the same file''.
That means you should have no problem with this. Qh4 is a check, so if Ke2 Qe4+ you should consider white gets murdered with Bc5+ On the other hand, Bg4 is not so much a threat, since there is plenty to stick in between in this position. So you know white will play g3 rather than Ke2. Then the rook is hanging in the air, which you should see. The double attack is then not hard to find. After you take the knight is hanging, so you can trust the queen will not get trapped.
You should see this with minimal calculation, if you analyze the position.
for me focus and attention took me a long time before i have improve it,playing blitz game started gradually increase my focus and attention,in addition to playing long games on tournaments,familiarization is a long process for chess, when you are familiarize with every different position,its only now that focus and attention is working properly to my advantage because i know now what is important is significant,tactics trainer may improve your focus and attention but not the time element for you to answer it or speed i should say,experiences and daily practice may improve your focus and attention.
Oct 14, 2011
0
#5
Try to answer this question : "if I was my opponent, what would I play to foil my own plans" 
Do it before playing your move, at every move. If you don't have time to do it properly, play a longer time control.
Hello, Chess.com, my problem in playing chess is how to keep your focus and pay attention to details. For some reason, in Tactics trainer, I only see the right move right after I moved the wrong piece or move. Here is an example: