Purely grammatically, こと in the first example sentence is a pronoun, and the second one can be interpreted both as a pronoun and a nominalizer.
In the first one, the subject("I") can't understand "the content you're saying" or "what you're saying", not "the fact/action that you're saying" or "if you're saying or not." Thus, this こと doesn't refer to the clause あなたが/の言っている itself. See the difference of following two sentences.
私はあなたの言っていることがわからない。
私はあなたが(何かを)言っているか(どうか)わからない。
As for the second example 僕が言ったことを取り消す, it can mean either "I take back what I said" or "I take back the fact I said (something.)" The former is the pronoun, whereas the latter is the nominalizer. The difference in the meaning is subtle, though.