Hello, Christine! ^-^
That's really true I think everyone has the same feelings with you as the members on italki.
I am also meeting to many person in here, and that's so you! ^0^
Thank you for correcting my English! So, I'd like to back your kindness this time!
When we try to write something in another language, everyone must have shy. because we know we will make mistake something then. But we know it's the chance to improving the language skills.
So, anyway, I'm going to read your sentence.

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perfect! even for writing. The thing of write is who can't make mistake even a little. It's a most severe skill than speak. But you did it just the same as like a native Japanese wrote!

Good! I won't say you made the mistake. However, I should tell you the best way to write.
Because your Japanese skills are high, I think.

not bad! If you spoke that phrase, a native Japanese won't be saying "pardon? what do you mean?"
However, for writing Japanese, it's going to be better you try to memorize another way.

try again I’d like to make sure once more that what you wanted to tell. Please try to write this phrase again, and we can tell you an exact sentence then.
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考えているんだ perfect! but Good!

You wrote it really perfectly. However, when famales write this phrase, it might better 

"考えています" 

The Japanese language, we have many ways to speak, actually.

Such as, Japanese females speak different words from makes. And also, we speak different words even same Japanese. It's depends where who live in. We have very different words and accents. 

So, sometimes, I don't understand what they are speaking, even a native. So, it's going to be better you read a Japanese female wrote a diary on a blog or is speaking something on Youtube.

ほぼ一週間前に「italki」のメンバーになって決めました。not bad!

ほぼ一週間前に「italki」のメンバーになることを決めました。

or

約（やく）一週間前に「itaki」のメンバーになりました。

その時から、たくさんの優しい方を出会っているんだ。not bad!

その時から、たくさんの優しい方と出会ってきました。

("I have been meating many good people since then.")

すでにずっとずっと習っています。 not bad!

もうすでに、いろいろ学ぶことができています。

本当に嬉しいです！ perfect!

助けています全ての人に感謝します。Good!

助けてくれているすべての人に感謝します。

本当にありがとうございます！ perfect!

まだうまく書きませんがだんだん向上しています（思います）。Good!

まだうまく書けませんが、だんだんと上達していきたいと思います。

頑張ります！perfect!

I did really good! ^-^

お互いに勉強がんばりましょう！！！