When you are reading sentences outside of the context, sometimes they can be considered awkward. I am thinking that you mean that everything in your life would be better after your brother's birth, but you were wrong about that.
However, the reader could interpret the sentence that you have written to mean that your brother's actual birth went wrong. In order to avoid awkward constructions, I suggest that you read sentences out loud to yourself and to another person who is not familiar with your topic. If they furrow their brows and make a confused face when they hear your sentence, they did not understand your intended meaning.
pohnpei397 has given you some excellent possible revisions for your sentence.
It is possible that the context is what makes the sentence awkward, because as it stands the sentence is fine. Context is always the most important consideration. I would recommend that you read at least two sentences before and two sentences after, in order to feel the flow of the paragraph. The is where the feeling of awkwardness may come into play.
Apart from this suggestion, here is another other way to write that sentence. You could break it up into two sentences.
"I was wrong about one thing. The birth of my brother did not make anything better."
I actually don't think this sentence is particularly awkward. It seems clear enough to me in that the meaning is quite obvious. If, however, you've been told that it is unclear and awkward and that you need to fix it, I'd suggest any one of the following:
- I thought the birth of my brother would make everything better, but I was wrong.
- I was wrong when I thought that the birth of my brother would make everything better.
- Although I thought that my brother's birth would make everything better, I was wrong.
I think Pohnpei's !st suggestion or sentence will be the most accurate.