AppleScript added value inside text
Text processing should be done with shell script. Usually it can be done in one short line:

sed 's;^[^0-9]*2 TEXT1;&amp; MY TEXT HERE;' text_file.txt


Assuming there wasn't some sort of indentation problem above. Based on what it sounds like you meant though, here's another solution:

sed 's;^[^0-9]*2 TEXT1;&amp;\'$'\n'' MY TEXT HERE;' text_file.txt


AppleScript is not the right tool for this job.