It's very painful when you don't recognize words you already know
When you see words you already know , but you don't recognize them cuz they're written in a different way
An example , the word 二人 (futari) which means 2 Persons
i saw it written somewhere as ふたり
i stared ! saying what is that ? i haven't seen that before
Till i went to the translator and knew that i already know it
it was very painful
It is very painful. (But it doesn't have to be!) As someone who has been learning Spanish for 10 years and Japanese for 4 (and still only barely conversationally competent in Spanish, nowhere near fluency in either language), I've experienced that feeling of "I should be better than this!" or "I learned this! I should remember it!" more times than I can count. But be careful.. "should" is one of the most painful and dangerous words there is, and not just in the context of language learning. Language learning should be fun, and the more you let your mistakes or what you can't yet do affect you negatively, the less fun it becomes, and the less you feel motivated to practice. If it becomes extreme, you can even set yourself up for failure. In situations like this, try to look at the situation from a different perspective. Like you said, you knew the word 二人! Great job! You just didn't recognize it, that's all :). It's the equivalent of recognizing an old teacher in a school building but not being able to place their face if you run into them in the grocery store. It doesn't mean you didn't know them. Context helps memory and comprehension, and that's why kanji is so useful and fascinating. (i.e. if you know what 二 and 人 mean, you know the definition of 二人 even if you've never seen the word before and don't know the reading.) I think because this situation happened to you, it creates an opportunity to begin to appreciate how much faster you can learn Japanese, and how much more you can understand written Japanese, by studying kanji. Congratulations on having a great learning experience! :)
I'm not trying to invalidate your frustration, and I agree it can be very cathartic to discuss the frustrating parts of language learning with others who can relate. This is a great post. I just felt compelled to warn you about using a self-defeating mindset too much because I've been there and it's really sabotaging. I'm learning Spanish because I want to understand everything, so whenever I don't understand things or can't say things, I get frustrated. I'm sad that I've been learning it for 10 years and I'm not better than I am. But I'm learning Japanese because I want to understand things in anime and manga, so whenever I understand something, it feels like a victory. I feel proud of myself because I focus on what I do know more than what I don't. As you can imagine, I'm enjoying learning Japanese much more than I'm enjoying Spanish at this point, but I'm going to work on shifting my Spanish mindset. Let's all enjoy our language learning journeys!
It often happens in Japanese, you have words that are mostly used with their Kanji forms and other with their Kana forms but I know that feeling. For example 随分 is mostly used with its kana form which is ずいぶん but it happens that you'll see the Kanji form and wondering "what the heck is that ?". One thing that is obvious while learning is, the more you practice the better you get. So don't be frustrated and say to yourself that now you'll remember it better, that's how you'll become fluent.
Some thoughts that may or may not be of any significance...
The reason you have/had no problems with 二人 is most likely two folded.
One is that 二人 is quite a vivid representation of the concept that these two characters represent, so the meaning is easily inferred.
Hence two, when your grey matter makes the connections with the meaning, it is not that long leap to get to the reading/pronunciation. You may think that the meaning and the reading are being discovered at the same time but I would dare to differ.
It's a different story with ふたり. In order to get to the meaning, your brain needs to have developed a different network of associations. If these associations are not there, or weak, you will falter. On top of that, it is highly likely that when listening to a real life conversation, or some speech being played back to you, you will not recognise futari and associate meaning to it.
The moral of all this is that, in language learning, it is equally important to make sure that your auditory cognition is on par with your visual one. Amen. :)