Here's the thing, though: Your explanations > the series itself's explanations.
My explanation is based on an interpretation of not only what is
said throughout the series about it but also what visual cues and actions have shown.
The "series itself's explanation"
has to be less accurate/defined than any fan interpretation made because
it doesn't give a definite explanation. The "series itself" leaves this topic ambigious!
Just because Mickey, Yen Sid, Eraqus, Lady Ava or Xehanort state something it doesn't mean that anyone of them actually has it right or that there even is an universal
right explanation.
In CoM Naminé as also an interesting tidbit of advice for Riku:
Naminé said:
Know that the darkness is there and don't give in.
Note how she doesn't propagate anything along the lines of embracing the Darkness (like Xehanort advocates) or going buddy-buddy with it (as Hades advocates and apparently some portions of the fandom think is what balance
has to mean).
Essentially what is propagated is to just say "oh hey, there you are Darkness. Yea, I know you're necessary so you can chill there, but otherwise fuck off!".
Yeah, I should've specified better: I meant resisting the impulse of doing what the negative emotions tell you to do (most likely something bad) rather than suppressing the emtion itself.
Oh, of course Sora and co. are way more resilient, but they're also the "heroes", the role models: he seemed to talk like acting like them is something everyone should've done, and that's much harder.
Yea, if one can resist those impulses or not of course also depends on the intensity of the negativity experienced in conjunction with one's own mental fortitude and willpower.
Being "blinded" by your emotions and losing the capability to act rationally as well as becoming obsessed with something is what has the "bad things" start happening.
Having already existing connections with others who might be able to help one bear and handle it better of course also play a crucial role in how each struggle turns out.
A person who has no support whatsoever may break and give in to the corruption way easier and with less pressure than one who does.
Acting like them is how everyone should
strive to do but obviously it is easier said than done.
In this vein I am actually pretty much admiring Terra, Aqua, Ventus and
Mickey since they went through so much shit of which big parts were actually shown (in Aqua's case even in more detail with 0.2) and they still persevered before Sora or Riku even entered the picture. One of the reasons why Terra telling Xehanort off in Blank Points and him summoning the golden chains in 0.2 with a "that's enough" is so impressive is because of this.
The fact that Xehanort had to almost literally
bombard Sora's heart with the pain and suffering of three or more other hearts and instigate another fight with Xemnas in DDD to finally wear it down also speaks for itself I'd say.
People "blaming" Sora too often overlook how massive the amount of "ammunition" Xehanort had to spent to succeed actually was.
If one really breaks it down though, while physical "torture" and force may certainly be an accompanying factor, I'd say that
most of the damage done in the KH series is actually mental, emotional and psychological.