Hey leannie, I looked into the issues surrounding fertility while breastfeeding (for the opposite reason to you!) and there seem to be a few 'conditions' to fully breastfeeding which help to keep you infertile. The main things were feeding at least once every 4 hours during the day and every 6 hours at night, and apparently you're more likely to remain infertile if you sleep in the same room as your baby, and aren't separated from them for more than 3 hours per day.
Here's the wikipedia page on it.
So I guess if you want fertility to return you need to try to space out the feeds a bit longer, and work on getting bubs to sleep through without a feed (try The Sleepstore for info on that), and maybe see if you can take a break from bubs during the day!