I need to work out what's making me sad. what can I do to work this out.?

That's a tough one. I had this issue a while back. Thought I had depression. Turned out it was a combination of little things. Maybe make 2 lists. One of everything positive in your life and one of everything negative. Then go through the negative list and cross out anything you have no control over. Then tackle the things you can control. Good luck.

yeh i agree with the above answer as well.
I think you should focus on what is good about ur life now, very easy indeed to say !
may be record what is going right in you life for a change, use some apps like instragram.com or calenstar.com to record one nice thing about ur day.