Now I understand a lot of things that maybe I didon't understooand before I started with this.

"dont understood" mixes present tense and past tense, if you're talking about the past, use "didn't + base verb"

All of this started when I left my ex-girlfriend, like a methodway to distract my mind, at this moment it feelingright now it still hurting mes, but less than before.

1. use a - for ex-girlfriend because it's a compound noun
2. method is ok to use but "way" sounds more natural
3. "right now" is more natural than "at this moment", "hurting" should be "hurts" bc proper tense and form and "feeling" isn't the right subject

Today is my fifth day writing oin my diary and I'll repeat the routine of every days.

1. "in my diary" because you're writing INside it
2. "days" is plural, but "every" always goes with a singular form

Now I understand a lot of things that maybe I didon't understooand before I started with this.

I am improveing with my English a lot, and I am very proud of this, i. In general, I am a better version of my self.

All of this started when I left my ex-girlfriend, like a method to distract my mind, a. At this moment, it feeling's still hurting me, but less than before.

Here, it's understood that "it" means the pain from your break-up.

Today is my fifth day writing oin my diary and I'll repeatcontinue theis routine of every days.

We write in books (notebooks, diaries, journals). But anything that is not meant for writing, we would say "on." E.g., "My daughter drew on the walls today."