XML : Preserving the Readability of XML Files

So I have an XML file:

  <diary>  <day1>  <line 1> Here is some text.</ line 1>  <line 2> Here is even more text.</ line 2>  <line 3> And so on...</ line 3>  </day1>  </diary>    

I now want to add personal comments to this XML file, say, for each line I want to write some sort of text, highlight a certain word, or write something at a very specific place in that line, e.g. between the words "is" and "some" in line 1. Or for example for line 1 I want to add something about the whole line: "In line 1 there is something very important, because here x happens" etc.

These comments can become quite lengthy and/or seriously modify the text. How would I add this to the XML file, without ruining the structure & readability of that file though? If I would add really long comments to every single line into the XML file itself, the file would not be as readable anymore. Same goes for highlights or comments inbetween words.

I would like to preserve the original diary entry though, without any comments or highlights, so I can maybe at some point in the future highlight something else, or read through the diary entry again in its original form. This is why I am quite reluctant to modify the XML file for this. Being a beginner, I am not sure what the solution might look like though.

Any ideas?