I think Ryouma needs a rest and be left alone with his slimes and have a vacation.
I don't think "fixing" is the correct term but I do understand the gist of what the main concern is. He's basically an 11 year old boy who's treating himself as a 30+ year old man. Yes, he has the mental experiences from his world and the other experiences did "transfer over" in a way but he has almost zero experience with actually, truly interacting with people. This isn't because he's an introvert, either. Even when he's tired, he's shown to enjoy working in the shop and being around people. What he basically needs right now is more experience interacting with others but that can't be forced. He's not "truly" balanced. He has good combat experience but poor people experience, to the point that MANY people have commented on it. This isn't a new issue.
The duke's family commented on it several times, in subtle ways. They commented on how he doesn't take care of himself or reach out for help. He understands his limits, but also doesn't. Him using his mana "until he gets tired" is pushing his body to his absolute limit. They worried about him overworking himself. They also commented on him under selling himself in regards to the dolls. He was too quick to overdo it with "gifts" and wasn't willing to let them pay for his genuine hard work. There's humility, yes, but it shouldn't be to the extreme of putting yourself down.
The various merchant heads have mentioned it, too. The one in charge of distributing his products has commented several times on him not valuing himself and needing to be a bit more proactive when it comes to pricing. On the one hand, saying "I trust you" in regards to that might be rewarding for some. On the other hand, it can be incredible pressure to live up to expectations to others. It isn't necessarily "nice" to leave that work to him and that's what he is essentially doing. The old lady is a bit more subtle in her commentary. Here's is more about catering to him but it all implies that he needs to work to earn everything. He isn't comfortable with gifts. This also shouldn't be normalized. All of them have been slowly introducing him to others, so he can have more interactions.
There are other instances but this chapter, 70, brings the main issue to the forefront. He doesn't value or trust himself. That isn't the only issue he has but this is a big issue in the current story. He's nice and thoughtful to others but at the same time, also careless in how he treats them. He's currently brushing it off as "earthling" thought process but he's been here for years at this point. Probably been about a year since he left the forest. And yet, while doing all these things for slimes or for others, he's kept his distance from this spotless. He's interacting with it but not acting like he's part of it. He's done no research into this world, to the point that common topics are all new to him. And it's finally being commented on.
He dealt with a LOT in his previous world and hopefully, he'll start addressing that soon. Maybe he can finally heal some of his issues with his father and his past life.
excellent assessment
But I also don't know how to fix what the mentor is taking about
Ryoma is such a well balanced all rounder that the only thing that would catch him off guard would be on the merchant side with a seemingly unhostile entity devising a drawn out scheme and to do all that for a lesson is not worth it
Maybe ryoma should say he's from the forest to clear the air a bit ╮(￣▽￣)╭