Tomorrow will mark one year since I wrote my first text here on LangCorrect. Tomorrow will mark one year since I wrote my first text here on LangCorrect.

Before that, I was mainly focusing on getting the hang of (the) basic grammar and developing my passive skills. Before that, I was mainly focusing on getting the hang of (the) basic grammar and developing my passive skills.

"The" is optional here and I think the sentence would sound a little more natural without it

Before that, I was mainly focusing on getting the hang of the basic grammar and developing my passive skills. Before that, I was mainly focusing on getting the hang of basic grammar and developing my passive skills.

Before that, I was mainly focusing on getting the hang of the basic grammar and developing my passive skills. Before that, I was mainly focusing on getting the hang of the basic grammar and developing my passive skills.

Passive skills?

Before that, I was mainly focusing on getting the hang of the basic grammar and developing my [passive] skills. Before that, I was mainly focusing on getting the hang of basic grammar and developing my [passive] skills.

Although technical words for the way we use language are used in formal settings (passive, active, and receptive, productive), for a general description like this, I suggest you name the skills (ex. my listening and reading skills). This gives your readers a more precise--and therefor more interesting--picture of what you were doing.

I am glad I didn't postpone this attempt any further, because, after gathering all the feedback to my posts for a few weeks, I became much more confident atwith/in building sentences. I am glad I didn't postpone this attempt any further, because after gathering all the feedback to my posts for a few weeks, I became much more confident with/in building sentences.

I am glad I didn't postpone this attempt any further, because, after gathering all the feedback to my posts for a few weeks, I became much more confident atin building sentences. I am glad I didn't postpone this attempt any further, because, after gathering all the feedback to my posts for a few weeks, I became much more confident in building sentences.

I am glad I didn't postpone this attempt any further, because, after gathering all the feedback ton my posts for a few weeks, I became much more confident ain/about building sentences. I am glad I didn't postpone this attempt any further, because, after gathering all the feedback on my posts for a few weeks, I became much more confident in/about building sentences.

It would be more colloquial to say, "I'm glad I didn't wait any longer to start writing." The prepositions in your sentence are a little tricky: I think that "feedback on my posts" sounds most natural in this situation; and "confident in" or "confident about" sound equally good to me in this sentence.

This allowed me to start speaking earlier, resulting in more rapid progress than usualI would have had otherwise. This allowed me to start speaking earlier, resulting in more rapid progress than I would have had otherwise.

This ending sounds clearer

This allowed me to start speaking earlier, resulting in more rapid progress than usual. This allowed me to start speaking earlier, resulting in more rapid progress than usual.

I think LangCorrect is one of the best things that have happened to me as a language learner, and I deeply appreciate everybody's time and effort that they have goneput into correcting my posts. I think LangCorrect is one of the best things that have happened to me as a language learner, and I deeply appreciate everybody's time and effort that they have put into correcting my posts.

I think LangCorrect is one of the best things that haves happened to me as a language learner, and I deeply appreciate everybody's time and effort that have gone into correcting my posts. I think LangCorrect is one of the best things that has happened to me as a language learner, and I deeply appreciate everybody's time and effort that have gone into correcting my posts.

I think LangCorrect is one of the best things that haves happened to me as a language learner, and I deeply appreciate everybody's time and effort that haves gone into correcting my posts. I think LangCorrect is one of the best things that has happened to me as a language learner, and I deeply appreciate everybody's time and effort that has gone into correcting my posts.

have = plural, you has = singular

I think LangCorrect is one of the best things that haves happened to me as a language learner, and I deeply appreciate everybody's time and effort that have---- gone into correcting my posts. I think LangCorrect is one of the best things that has happened to me as a language learner, and I deeply appreciate everybody's time and effort that ---- gone into correcting my posts.

The second part of this sentence has an interesting challenge. On the one hand, it's acceptable grammatically for you to use "have" because you think the verb should be plural since there are two subjects (time and effort). On the other hand, the reason Tolu's correction stands, and I agree with it, is that "time and effort" is a set phrase, a unit that goes together and takes a singular verb. Of course, you could also reword the statement: "I deeply appreciate the time and effort everyone has put into correcting my posts."