Create A Table Which Contain History Of Main Table

I have to create a table which contain history of a main table. like this:

if the main table is
========================
nametypelengthnot null
Avarchar5Y
Bvarchar5N
Cvarchar5N
Dvarchar5N
========================
[code]....

I've plan to so this by create a trigger in main_table. my problem is my main table have a lot of fields and I can't write a code to control it 1 by 1 like :

if old.A <> new.a
insert into history("A",old.A,new.a)
if old.B <> new.B
insert into history("B",old.b,new.b)
......

I decided to select column name from the data dictonary using this SQL:

SELECT column_name FROM user_tab_columns WHERE table_name = '<<Table Name>>';

and then do a loop over the resultset and use the column name I've got , like this (its just an idea, may be not a write syntax):

BEGIN
.....
FOR i IN 1..:result.COUNT LOOP
if ld.colname[i] <> :new.colname[i]
INSERT INTO history
VALUES ( colname[i], ld.colname[i], :new.colname[i]);
END LOOP;
END;

but I can't write a "old.colname". I try with " old.'colname' ", " ld.'colname' " but it won't work.how to create a history file like I've describe.