How can I send plain text email (with line breaks) using sp_send_dbmail?

You aren't actually inserting any line breaks. You can embed them directly in a string literal in SQL Server as below.

SET @bodyText = (SELECT N'Here is one line of text 
It would be nice to have this on a 2nd line 
Below is some data: 


' + field1 + N' 

' + field2 + N' 

' + N'This is the last line'
                 FROM   myTable);

Or a tidier approach might be

DECLARE @Template NVARCHAR(max) = 
N'Here is one line of text 
It would be nice to have this on a 2nd line 
Below is some data: 

##field1##

##field2##

This is the last line';

SET @bodyText = (SELECT REPLACE(
                    REPLACE(@Template, 
                       '##field1##', field1), 
                       '##field2##', field2)
                 FROM   myTable); 

Both will raise an error if myTable contains more than one row as you are assigning the result to a scalar variable.

I've always used CHAR(13)+CHAR(10) to create line breaks (which seems to work mixed in with nvarchar values) in TSQL, so try something like this:

DECLARE @CRLF char(2)
       ,@bodyText nvarchar(max)
       ,@field1  nvarchar(10)
       ,@field2  nvarchar(10)
SELECT @CRLF=CHAR(13)+CHAR(10)
      ,@field1='your data'
      ,@field2='and more'

set @bodyText =
                N'Here is one line of text ' 
                +@CRLF+ N'It would be nice to have this on a 2nd line ' 
                +@CRLF+ N'Below is some data: ' + N' ' + N' ' + ISNULL(@field1,'') + N' ' + ISNULL(@field2 + N' ' ,'')
                +@CRLF+ N'This is the last line' 


PRINT @bodyText

OUTPUT:

Here is one line of text 
It would be nice to have this on a 2nd line 
Below is some data:   your data and more 
This is the last line

this CHAR(13)+CHAR(10) will work with msdb.dbo.sp_send_dbmail, I send formatted e-mails using that all the time.