Only thing i can say is to play with confidence in every decision that you make, to the point where it may seem cocky. Ofcourse there will be mistakes and wrong decisions, thn simple try another answer and play with the same confidence. It’ll make you play more relaxed. Ofcourse experience and a lot of other stuff counts though which will back up this confidence, but let it be a gradual process. Dont understand something, go in training and figure it out, and while playing it’ll come naturally. When you are stressed you cant keep focus and wont be able to play to the best of your abilities.
Not saying this is the best or right answer but ive noticed all the best players play in such a way, no hesitation, just absolute confidence, its noticeable even in the small things in how theyll walk or pick spots on the stage(obviousely they know they ranges or mindset of their opponent that allows them to press buttons at right times or know they are hesitant/scared which dulls reactions and allowing them to walk more forward. Than normally).
This way you are able to keep focus during the match, however do keep an eye on the oppon3nt very clearly, use the knowledge gained from watching them, what are their habits, their timings, their strengths and their weaknesses and play accordingly.
Lastly, have a clear general gameplan, a plan against new players, to test them at every moment and gather that knowledge and use it.Also have a clear gameplan against them for the matchup itself. Where to stand, what to look out for, what and how to punish. If you actively keep yourself busy with these things instead of fretting about when you are starting to lose you are working on a productive way to turn the game around… and you’ll keep focus.