I'm talking about reduced to tears level frustration, with someone trying over and over again with no improvement to learn a skill(this case sword fighting) they're under pressure to learn but just... can't for some reason. However, I worry over it coming out as a bad type of angst(something I've done before to my massive regret) so... tips for portraying it? Have you done this before, and if so, how did you do it?
Maybe you could let the reader "feel" the frustration. As in, rather than have the character sequelising about how awful he feels about his ineptness all the time, show him get angry when he gets beaten by his sparring partner, refuse to talk about it, break stuff in frustration and then storm off. Also, what do people around him think? Are they treating him with disrespect because he can't master this skill? What is the reason that he can't get it? Is he a pacifist at heart, an artist? Does he have a physical impairment (impaired vision for example would make sword fighting difficult). Does he lack the motivation to become an accomplished swordsman? You can also show him getting depressed, but not writing his thoughts, rather, portray him as someone getting increasingly irritable with people who love him, withdrawn, a loner etc. And then have him open up in a flood of tears to someone (maybe in a bar, when he is drunk) about how miserable this makes him feel, but add some context to it (does he feel he has disappointed his parents, or that he is letting the village down, or that he is not man enough etc, whatever makes sense with respect to the story). That's how I would do it.
Ah, interesting. Well, in this case... it's because the entire world depends on her to be able to fight with a sword well.
Imagine the pressure! Well, then my suggestions need serious reworking because I assumed the character was male. To react in this way would not be quite consistent with a female character. Hm... I can see your problem. Well, ok, imagine the pressure on today's girls to be thin. Then trying to comply with the society's demands against their own nature, and growing bitter about it. On one hand, it is so hard to starve yourself and reach the goal of perfect thinness (good proficiency with the sword), but on the other - how weak am I to even give into this expectation. In response to unreasonable demands, severe criticism and pressure, girls often develop eating disorders, they become either control freaks (anorexic type) or poor impulse control (bulimic type, unable to control the urges but feeling extremely guilty and self-depricating afterwards, indulging in risky behaviours etc). So maybe you can see what fits better, for her to become a control freak, who denies that there is a problem and punishes herself when she fails, or the other extreme where she acts out a lot and feel helpless to affect her destiny. In both cases, she won't want to admit that she has self-esteem issues so you'll have to "show" the frustration rather than talk about it. Like Tim Burton's Alice in Wonderland, she had great internal conflict without going on about it too much (when she was supposed to defeat jabberwocky, with a sword no less).
Well... she isn't quite your typical female character, and actually didn't even start female. Okay, its a long story, but basically she's a guy who got punished to be female and has to go kill someone who can only be killed by being bested in single combat. It... causes issues to say the least. Okay, sorry for not warning of the first, as I realize this... complicates things. But thanks, you've helped a lot already.
Ouch! This might be very controversial I am glad it helped a bit. It is the knowledge of various personalities (as in personality profiles) that helps with figuring out how a character would react to a given situation.
Um, no offense, but is this a good idea? It's a very strange situation if the fate of a whole world depends on one person's swordfighting ability. Can't she get some help from somewhere? Isn't there a better plan than just to put her out there on her own? Hmm.
In of itself? No. However, the story is actually a critique of destiny, and us assigning roles to people in society, like in this case someone being a heroine. Now, don't worry, she does get training for this, as alluded to in the OP, and the person training her helps her a fair amount.