Wow, I feel for you... But kudos on being a good guy and respecting her wishes despite how hard it is.
One thing that might help her understand how safe from pregnancy you really are on the patch/condoms is if she tried keeping track of her temprature. When women ovulate they have a temprature spike, women who are on the patch/pill don't ovulate, maybe if she kept track for a month or so she'd see that she's not ovulating and therefore cannot become pregnant.
Also, she may simply not be that educated as far as how well protected she is. Take her to planned parenthood or a similar agency and get some info for her to read, maybe once she knows all the facts she'll feel better about it.
Thirdly, you could always throw a third method (or fourth), of b/c to make her even more comfortable, try a sponge or spermicidal foam or jelly.
Finally, I hate to say this but she may have other issues with sex and she's just scapegoating. If none of the above things satisfies her, this may be the case. Sex can be very tramautic for some people, and there do exist people who are asexual, and simply have no desire at all. Talk to her and try and make sure that fear of pregnancy really is the cause.
Best luck to you,
~laura