I have been writing one morea journal oin my notebook for more than 50 days.

Now that I tryI am trying to write it in English, it helps mye thinkings straight into English.

Adding "straight into" isn't really necessary here.

Today, I fiound out that I need to buy another one.

It would be better to say "I realised that...."

I want to use this brand's notebookbuy a notebook from this brand again, because I have already geot used to it.

On the other hand, I also want to usetry other brands which I haven't used.

Since you have not used them yet, you would use "try"

For this reason, I have browsed on the many internet shopmany websites, but I do notam still not decide a next notebookd on which notebook to get next.

I shouldwill think about it again tomorrow again.

It's more natural to say "I will.." as you are (most likely) going to do it

I have been writing one more journal oing in my notebook everyday for more than 50 days.

Now that I try toam write iting in English, it helps my more often, I am able to thinkings straight into English the language.

You could also say "Journaling in English daily has helped me think directly in the English Language."

Today, I fiound out that I need to buy another onenotebook/journal.

You could also say "Today I realized that I need to buy a new notebook/journal."
It is better to restate that 'another one' is referring to the notebook since the previous sentence was about a different subject.

Because the notebook has onlyit only has 60 days' worth of paper.

Since I edited the previous sentence to include 'notebook', in this sentence we can refer to the notebook as 'it'.

I want to use this brand's notebookbuy from this notebook's brand again, because I am already get used to it.

You original sentence could also be correct with a minor fix like so: "I want to use this brand's notebook again, because I got used to it".

On the other hand, I also want to use otherwould also like to try new brands whichthat I haven't used before.

Check out an explanation on using that vs. which. To be honest, this topic can be pretty confusing, so don't focus too much on it. When editing this sentence, I did not think about the grammar rules, but rather what I felt was more natural.

For this reason, I have browsed on the many internet shops, but I do not still decide astill haven't yet decided on the next notebook.

I should think about it again tomorrow again.

I think your original sentence can be correct, however I edited it in the way that sounds more natural.