This is my first time offering advice, but the following works for me:
1) Set up a Data Connection that points to your SharePoint list. When
you name your collection e.g. "List" it will show up as a secondary
data source in the data source task pane. There is a dropdown that
toggles between data sources.
2) Create your rich text box, then rightclick and select "change
binding." This will bring up your primary data source but you will see
another dropdown in this dialog. You will be able to bind to the
secondary data source.
For step 1, you will have to say, retrieve data when the form is
opened. Alternatively you can trigger this retrieval from a rule.