I have been learneding English sincefor fifteen months ago, after changing my learning method. I have been learning English for fifteen months, after changing my learning method.

You can also write: “after I changed my learning method”.

Two years ago, I just played 'Duolingo' app which is a kind of language learning app. Two years ago, I just played 'Duolingo' which is a language learning app.

You can also say, “I just used “Duolingo”, which is a language learning app”.

WThen I watched a video on YouTube, and it hit me. Then I watched a video on YouTube, and it hit me.

The video was about “how to teach kids English to kids” and I thought that the method was good for me, too. The video was about “how to teach English to kids” and I thought the method was good for me, too.

Like the guide otips shown in the video, I have spoken children's book loudly. Like the tips shown in the video, I have spoken children's book loudly.

Or “Like the guide in the video suggested..”

Like the guide on the video, I have spokenread children's' books out loudly. Like the guide on the video, I read childrens' books out loud.

While this suggestion doesn't preserve the emphasis on how loud it is, the way your sentence reads makes me think that the act of reading books out loud is the important part. I'm also assuming you read more than one book, if I'm not mistaken. That's why I changed it to "books"

After 10 months later, I felt confident about English, because I could read English books more easily and I could listen to videos which speaksthat are in English. After 10 months later, I felt confident about English, because I could read English books more easily and I could listen to videos that are in English.

Now, I still have problems aboutin the writing and speaking in Englishparts, but I will do my best to overcome these problems. Now, I still have problems in the writing and speaking parts, but I will do my best to overcome these problems.