I Llike Wwriting

Titles are not all capitals.

I had thean online English lesson this morning.

Now I'm passionate abouto learning English, especially writing.

I had thean online English lesson this morning.

Because the English lesson was for you personally, we say 'a' or 'an'. If the lesson was something everyone had and wasn't temporary, we might use 'the'.

Now I'm passionate abouto learning English, especially about writing.

What you wrote feels correct and people will understand your meaning if you say it this way.
Another way to write it is using 'about' to focus on what you mean. Here's an example: I'm excited about eating a large meal tonight.

I need to learn more grammar, because grammar is my weak point.

The word 'because' strongly focuses attention on what comes after it.
Here, you write two ideas/clauses (I need to learn more grammar | grammar is my weak point).
You can connect these ideas in many ways but 'because' strongly connects ideas and a comma doesn't always do so.
Using commas can be tricky because they have many uses.

Here are some examples of ways to write this sentence:
1. I need to learn more grammar because grammar is my weak point. (using 'because')
2. I need to learn more grammar, it is my weak point. (using a comma)
3. Grammar is my weak point and I need to learn more about it. (re-ordering your ideas)

So, I will try to read a grammar textbook again.

This is technically correct, but 'so' is usually used for connecting this idea to a longer sentence.
Another way to connect your sentences and have them flow is by using a semicolon (;). A semicolon connects two sentences that are related.
Here's an example: I purchased some vegetables; I'll add them to my soup.