This is a tough question. It really sounds like the problem is not the GMAT. The problem seems to be nerves on test day. If it is purely a case of nerves, I don't know if taking a different test will help all that much. That said, you could take a free practice GRE and see if you find it a lot easier. The quant section is certainly easier, but the vocab piece will be new.
If that doesn't seem like an option after you take the test, you will need to find a way to convince schools that it is not an issue with the academics. It is an issue of test-taking. You can do this, by showing explaining in the optional essay they usually provide and demonstrating elsewhere that you can handle the course load. You can do this by pointing outstanding grades in undergrad or by going out and taking a quant related course now and scoring very well. None of this is ideal, but schools do realize that there are a handful of people who don't do standardized tests well. Of course, you will need to do an artful job of telling your story and showing your strengths, but it can be done.