Quentin... You are correct. Sometimes we simply say "so" by itself.
"So" can have a similar meaning to 'to that extent' or 'that much' or 'very much'.
It is often used when we are talking about a high degree of some quality (like love or hate), and in situations where 'very' is also a suitable word.
"I love you so" = I love you so much / I love you very much / That is how much I love you /
That is the way (in the manner previously mentioned) I love you.