Dont misaunderstand me: I had a lot of fun, learned a lot and met nice people who bravely read through my mistakes. Dont misunderstand me: I had a lot of fun, learned a lot and met nice people who bravely read through my mistakes.

Probably a semicolon would be better here?

But I overestimated myself. But I overestimated myself.

I shouldn't have tookaken a challenge so soon. I shouldn't have taken a challenge so soon.

"take" is an irregular verb, its past participle is "taken".

I also had other obligations that took a lot of my time, and more work everyday just didn't cufit in. I also had other obligations that took a lot of my time, and more work everyday just didn't fit in.

I don't understand what you mean by "just didn't cut in". Maybe this?

As I'm fFrench and on this site, you had probably guessed ithat. As I'm French and on this site, you had probably guessed that.

"that" is more natural here. Can't explain why.

The idea behingd Langcorrect seemed sound. The idea behind Langcorrect seemed sound.

I have to push myself every day to do it. I have to push myself every day to do it.

Very common error; "everyday" is an adjective meaning "ordinary, commonplace", and isn't technically correct to use when you're saying something occurs every day.

It's actually harder to write about mundane things than to writinge stories. It's actually harder to write about mundane things than to write stories.

Infinitive for both or gerund for both here. "Writing about mundane things is actually harder than writing stories."

I didn't know that. I didn't know that.

The problem was that I was already writing 1400 words in French by day, working on my short- story and editing older texts for publication. The problem was that I was already writing 1400 words in French by day, working on my short story and editing older texts for publication.

I've never seen anyone hyphenate short story.