I recently posted a thread about overcoming Erectile Dysfunction. I feel your pain, man.
Although they didn't work for me due to the severity of my issue, a lot of the natural supplements I've tried will probably work for you.
Look into Yohimbe and/or mixing Pycnogenol with L-Arginine/L-Citrulline (they must be taken in conjunction).
You might want to try Yohimbe first, as it will be easier to find and less harsh on your wallet. It's a "natural supplement" so it can be found at most stores -- I got my bottle of 30 at Rite-Aid for $7, and it's the exact same shit they'd charge you way more for at some health foods store. CVS and Walgreens might carry it too, but I'm not too sure.
The Pycnogenol + L-Arginine/L-Citrulline combo would be more of a backup plan if the Yohimbe alone doesn't do the trick... and even if the Yohimbe does work, taking it along with this stack is like giving your dick the serum they used to make Captain America.
Yohimbe -- take about 30 minutes to an hour before sex. It's effects last twelve hours, however, so timing isn't too important.
The other combination of supplements -- take in the morning, at the start of your day.
EDIT: If you find one pill of Yohimbe too intense for you, such as causing side-effects like anxiety or tension, get a pill splitter and take half of one the next time. It will still be effective, but without the side-effects.