はじめまして

Good job!

Ending the essay with すみません is very Japanese. Because Japanese people are apologizing all the time.
In this essay すみません appeared a  bit too sudden though. So, I think you should add something like above meaning;

I am glad to meet you. I'm Lilli.  I'm 14 years old. Japanese is difficult. I am sorry that I am not  used to it yet.

はじめまして

Yuki-san explained well about Sumimasen.

i would like to point out私の日本語は大変だね doesn’t make sense.

It is either  私の日本語は変だね。 (My Japanese isn’t right )  or 日本語の勉強は、大変だね。(It is hard to study Japanese)

You might have meant私の 日本語は、とても変だね。

Liliさんの日本語は変ではありませんよ(おかしくありませんよ)。

よくかけています。じしんをもってください。