What is wrong with this (from a novel)? "Which I wasn't saying to be nice, it's completely true."
You can't start a sentence with a "Which" clause like this.
You can't use contractions like this.
The punctuation is incorrect.
Nothing is wrong.
Congratulations to 29% of you.
You can start a sentence with a "Which" clause. For more info, see this Grammar Girl episode.
As for the contractions, they're fine.
The punctuation, however, is not fine. You can't just add a sentence to another. You have to add a period after "nice."