>>395
When 真面目 qualifies a noun, you put な at the end of the word. So if it reads 私は真面目な日本語の学生だ。, it's acceptable. But the sentence you got on lang-8 sounds more natural.
As far as I know, both are grammatically correct and a native speaker would say in both ways. But for some reason the latter seems more frequent.
>>396
If you put 我ながら or 自分で between "なれば," and "驚きます," the sentence becomes ok. The two words mean exactly the same thing in this case and sound formal or informal respectively. If you omit 僕 in addition to this, i.e., if it reads 何時か上手になれば、我ながら (or 自分で) 驚きます。, it sounds perfectly natural.
The correction you got on lang-8 is gramattically ok, but has the different meaning; you meant surprised, not happy.
If you wonder why the latter sentence doesn't need 我ながら or 自分で, it's because feeling happy is natural when you get better at something while surprising at yourself implies sort of modesty or humility, and you should use some kind of modifier expressing your self-effacement, which is 我ながら/自分で in this case.
It's not necessary to omit 僕 when you say "...嬉しいです," but it's better not to use it when you put 我ながら/自分で because they clearly indicate who becomes skillful.
On a side note, if you don't want to sound too formal or too informal, you can say 何時か上手になれば、自分のことながら驚きです。. 驚きます is also good and means the same.