Try something like this:
procedure TForm1.AddFormattedText(const AText: string; AStyle: TFontStyles);
begin
RichEdit1.SelStart := RichEdit1.GetTextLen;
RichEdit1.SelLength := 0;
RichEdit1.SelAttributes.Style := AStyle;
RichEdit1.SelText := AText;
end;
AddFormattedText('Test', [fsBold]);
AddFormattedText('123'+sLineBreak, []);