I'm not sure you lost because of your mentality. I think you played positively and did a few nice shots at the end, notably some difficult BH
However, you made too many mistakes, unforced errors especially on FH opener. Maybe you should consider trying to play more safely. a slower opening loop, but put in on the table. It is a mistake to put too much power systematically in the 3rd ball attack, or - worse- to look for a winner immediately.
You should have in your arsenal more serve+3rd attack combos. It looks like your serve is not tight enough or with enough spin, to guarantee with a higher probability a return where you expect it. I thought your timing for the topspin was a bit late, under the table which makes it more difficult to attack when near the table (which is the case of 3rd ball attack)
I thought your receive were a bit too long and passive as well.
In this particular video example, i would argue you didn't lose because of lack of focus or mental problem, but because of technical problem.
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More generally, i'm fighting as well hard to improve the mental aspect of the game. I think i've improved here. First of all, i had to recognize its an important aspect of the game, which needs training as well. Like you work on your FH with drills, you train mentally when training, by learning to focus 100% of the time, even when drilling, blocking for your friend, playing a match against any player, even someone of a much lesser level. You can also learn relaxing techniques, take a deep breath from time to time, or relaxing a tense muscle; Also learn to always think positive, even when you miss a shot or you're down a lot. Never let anger or despair take the better of you.