So I think our ultimate diagnosis is different. I see in OP a player who got to 1900 and misunderstood what the 1900 rating was because of how he got there, and assumed that 1900 earned after playing for a few tournaments (say 20 matches or much less) was the same as 1900 earned after playing for a few years of tournaments (maybe 200 matches, at least 50 or more). With the latter, you have usually competed through a variety of circumstances and developed a solid understanding of competition. With the former, the context in which you earned your rating can be very incomplete and you might be too low or too high depending on the context. It is much more difficult to have the wrong context having played almost 200 matches though it is theoretically possible. This is one of the criticisms of the USATT ratings system that Ratings Central tried to fix by using more statistics to determine a full rating picture.
It's influencing his mental strength now, but it is partly because the original rating gave him a wrong understanding of table tennis strength. To become a solid "whatever rating" player, you have to have competed in multiple circumstances against a variety of styles at that level to be truly solid. And this is notwithstanding the variations that will occur with age and health and mood etc.
I'm trying to build some players right now and with my coaching, one of the things I try to keep them aware of is the context in which my coaching and the skills I am showing them work and the contexts in which they don't, so they stop seeing table tennis as an "I can do anything because of my skills game" and more as a "this things need to be in place so I can do this, and I need to find a way to get the opponent to play to my strengths"). Being strong against topspin players doesn't mean you won't struggle against pips, I have seen 2300+ rated players lose to 1800 pips players, its part of the reason I am big on early pips exposure for learners so their understanding grows with their level of play.
But someone starts missing the table because they like to play fast all the time, and I try to explain to them that when facing pips, there is no need to attack hard until the opponent imposes that tempo on you. Or someone gets a kill shot opportunity and focuses exclusively on power and less on placement and gets the ball blocked back. Or someone can return pendulum serves at 2200 level but backhand serves to his short forehand at a 1700 level but doesn't address the gap and then wonders why once his opponents know what to do, they consistently beat him.
Long story short, OP definitely achieved the 1890 rating but as to whether he had all the skills truly associated with 1890 and whether defending that rating for him is all about mental strength, I am not sold. Not that there are specific skills per se associated with a rating, but more that sometimes, you need the struggle to truly build mental strength, and we sometimes pretend the strength is easy or possible to build apart from the struggle.
Because of that distortion, he sees his losing as a sign he is not playing his best. Whether that is true or not is an entirely open question.
