I wanted to write something sooner, but I was quite busy atduring the weekend.

You could also specify which weekend and replace 'the' with 'this' or 'last'.

Maybe I could have found thmade time to write a journal, but I was either too tired to do so, or I didn't even get the ideaknow what to write about.

You could add 'didn't have a topic etc... to write about'. I believe time is sort of uncountable here (in the correction).

I wanted to keep my streak and reach asthe highest number as possible, but I struggled to find the time every day and, more importantly, topics to write journals about.

Struggle in present tense is fine as well. Although you may find some teachers picky about it (tense consistency)? I know where you're coming from though.

I wanted to write something sooner, but I was quite busy at thethis weekend.

You could also say "I was quite busy over the weekend," but never "at the weekend."

Maybe I could have found the time to write a journal entry, but I was either too tired to do so, or I didn't even get thehave any ideas.

I wanted to keep my streak and reach asthe highest number aof days possible, but I struggle to find the time every day and, more importantly, I struggle to think of topics to write journals about.

I understand what you mean by the last bit ("more importantly..."), but the way I rewrote it sounds a lot clearer.

I suppose it's not that long for experienced hikers; however, notit's long for me, because hiking is something I don't do very occasionallyften.

The combination "very occasionally" is not one that I have seen before.

On top of that, I had a headache that lasted all night.

That was probably the ultimate reason for writing no journal at almain reason I didn't write a journal that day.

An English idiom that might be helpful: "the straw that broke the camel's back." This blog post provides a good explanation...
http://eslidioms.blogspot.com/2014/12/the-straw-that-broke-camels-back.html

On Saturday and Sunday, I participated competitions in fire sport competitions.

You'llI'd be surprise med if you do!

What you wrote is correct - my suggestion is just a more idiomatic version.

I wanted to write something sooner, but I was quite busy at thethis weekend.

Maybe I could have found the time to write a journal, but I was either too tired to do so, or I didn't even get thehave any ideas.

“Get the idea” is a phrase used to reflect understanding. Ex. “Do you get the idea?” is an informal way of saying “Do you understand?”

I wanted to keep my streak and reach asthe highest number as possible, but I struggle to find the time every day and, more importantly, topics to write journals about.

On Saturday and Sunday, I participated competitions in fire sport competitions.

You might not know this sport, as it is specific to the Czech and Slovak Republics.

Maybe I could have found the time to write a journal, but I was either too tired to do so, or I didn't even get thehave an idea.

I wanted to keep my streak and reach as highest a number as possible, but I struggle to find the time every day and, more importantly, topics to write journals about.