Rather than typing all this into Calibre you may have more joy with the following approach.
Edit your source .TXT file's headings using
Markdown markup language, like this:-
Code:
# My Book Title
# My Book's Author
///Table of Contents///
## SOMEONE LIKE YOU
... blah blah blah ...
## Taste
... blah blah blah ...
## Lamb to the Slaughter
... blah blah blah ... etc etc
The single # lines will be treated as <h1> and the ## lines as <h2>.
The ///Table of Contents/// line should be placed where you want the internal TOC to be.
When you've finished editing, convert the TXT to EPUB, making sure you check the [Convert] - [TXT Input] - "Process using markdown" box.
If you would also like a TOC which appears in the TOC left sidebar in the ebook viewer you should also do this during the conversion
in [Convert] - [Structure Detection] set "Detect chapters at" to
//h:h2
in [Convert] - [Table of Contents] set "Level 1 TOC" to
//h:h2
You can read more about markdown
here