I'm the same way man. Not only the rock hard factor, but after contact stops, so does the erection. I'm a reasonably healthy eater, take my vitamins and all the supposed herbs that help (gingko, turmeric) and no avail. Not to say your case is as bad as mine, but I even went to a urologist, and was given both cialis/viagra, once again no avail.
My only advice is to try and not think about it, and talk to your chick about it. If you let your mind to get to hyped and worry about shite like this, it will just make things worse. If you are old enough and not shy about things like this, a cocke ring might help out.