If you don't care about any instances of a period that is not used to end a sentence. (i.e. Mr., Mrs., i.e. ...)
then you should just be able to replace all instances of periods in your document with a period-newline combination.
doc = doc.replace( ".", "." + Environment.NewLine );
If you write this resulting string to a textfile, you should have your one sentence per line done.
NOTE:
You won't see the results properly in your Textbox unless you turn the Multiline property to true.