Only you have this information. Sit down with pen and paper and jot down as much as you can remember. Once you start, it becomes easier to fill in the gaps. If you had a period of unemployment, just state that with the dates.
It is only 12 years and for Agency work, you could check to see if the Agency kept records of your time there.
Do you definitely have to go back 12 years, usually its a 10 year checkable history?
Good Luck.