I will try to answer this, though it is a tricky question even for a native speaker. "To me" is often another way to say, "in my opinion". This is the case in your example, "This text sounds natural to me." You could give the same meaning by saying, "This text sounds natural, in my opinion", or "In my opinion, this text sounds natural." They all mean the same. In your A and B examples, I would be inclined to use "For me", though it is acceptable to say "to me", because it is your opinion that the text is difficult. With "for me" you imply the text is difficult "for you" to complete some action related to the text. For example, "Understanding this text is difficult for me to read," or "Understanding this text is difficult for me to see," or "Understanding this text is difficult for me to comprehend." In your example using "for me", the final infinitive verb is omitted. It is definitely acceptable to begin a sentence with "Understanding.....", but it cannot stand alone as a sentence. It is a subordinate clause that needs a subject. For example, "Understanding this text is difficult for me, I will read it several times." I hope this helps! Anne