However, I only learned for real when I kind of failed with Japanese.

Consider removing "Kind of", it breaks up the sentence's flow and sounds unconfident.

The biggest mistake was self-sabotaging while reviewing the flashcards.;

You seem to split up sentences a lot, try to form them into one.

When the word showed up months later, I was not able to get it right because I didn't trully learned it.

Now, the cards show a clozse deletion sentence, along with the translation of the missing word or a definition in Spanish of the word.

Check spelling, I'm not sure what you wanted to say there.

Years ago, I bought a course that teachedaught me how to learn languages.

button even not being 100% sure if I got it right.

This led me to forget words in the long term.

When the word showed up months later, I was not able to get it right because didn't trully learned it.

I think I've already wrote a text about it so I'm just reinforcing it.

I'm not sure what you meant by "text" - did you mean "post"?

Instead of testing myself to guess the meaning of a Spanish word toin my native language, I changed the flashcards to production cards.

I also want to go back to studying Japanese asfor a second try.

You can also say "I also want to give studying Japanese a second try".

Years ago, I bought a course that teachedaught me how to learn languages.

button even not beingthought I was not 100% sure if I got it right.

When the word showed up months later, I was not able to get it right because I didn't trully learned it.

Using Anki to learn English has been much different thano my Japanese experience.

Secondly, because I switched to another approach ton using the software.

I also want to go back to studying Japanese asnd give it a second try.

For instance, I used to press the "I guessed right!"

Maybe I would say something like "I used to press "Good/Easy""

Not sure how it is in other languages, but at least on the English version of Anki, when you have a new card, there are 4 options. Again, Hard, Good, and Easy. Good sets the interval to 1 day and Easy sets it to 4. There is no "I guessed right!" button in Anki.

When the word showed up months later, I was not able to get right because diI hadn't trully learned it.

Firstly, because English is easier for me compared to Japanese.

Secondly, because I switched to another approach ontarted using the software differently.

Instead of using "secondly" here, (and "firstly" in the previous sentence) it might be better to say "The first reason is...." "The second reason is...."

I think I've already wrote a text about it so I'm just reinforcing it.

I'm not sure what you are "reinforcing". You should probably have clarified more here.

Instead of testing myself to guessbased on getting the meaning of athe Spanish word toright (by recalling the translation in my native language), I changed the flashcards to be production cards.

A bit more of a technical term, but in the language learning community some people say they "grade themselves" based on certain criteria. so "Instead of grading myself based on getting the meaning of the Spanish word right (by recalling the translation in my native language), I changed the flashcards to be production cards."

The word "testing" is also totally fine here though.

Now, the cards show a cloze deletion sentence, along with the translation of the missing word or a definition in Spanish of the word(into my native language) or a definition of the word written in Spanish.

A good word to use might be "monolingual" if you mean that sometimes you put Spanish definitions of the Spanish words. You could say you use "monolingual" Spanish definitions, because they are Spanish->Spanish

I also want to go backive studying Japanese as nother try (or "a second try").