I read somewhere (sorry, can't remember where) that the chances of becoming pregnant in your 40s are not as low as has been published and assumed. I know that in the link ChillDoubt posted, it said there is a 5% chance in each unprotected menstrual cycle for a woman in her 40s, but it doesn't mention the equivalent figure for women in their 20s. I believe (from what I read) that it's not as much as you might think.
As you have been trying for 2 and a half years, perhaps it's time to see your GP if you really want another child.