In my view, sentences inside "(" and ")" are annotations.

Do not annotate sentences.
http://blog.tatoeba.org/2010/02...eba.html#rule9

You may be interested with my translation:
http://home.ustc.edu.cn/~sadhen...in_Tatoeba.htm

I don't think this is an annotation... here, it's just part of the sentence. These are examples of annotations:

=> Alex (male) is my boyfriend.
=> He does not (doesn't) know where the keys are.
=> I like him/her.

I think there should be a colon after "by believing this forever" instead of a comma, and possibly quotation marks.

What's more, from now on you must remember that you cannot go to the seaside again (by believing this forever, I have already found the biggest, best and most fitting one for me).
->
What's more, from now on you must remember that you cannot go to the seaside again. By accepting this forever, I have already found the biggest, best and most fitting stone for me.

Does this match the Chinese?

It's more like this:

What's more, from now on you must remember that you cannot go to the seaside again. (Believe this forever: "I have already found the biggest, best and most fitting stone for me.")