@slidingsideways is allergic as well and takes some kind of regimen I can ask her about that seems to mostly keep it under control.
I take generic Allegra in the morning but my game changer is Singulair (prescription; generic is montelukast). It's technically an asthma medication, but it also lowers the body's total IgE level, the antibody we produce in response to allergens, which is like turning down the volume on all of my allergies. Full disclosure: Singulair has a warning for psychiatric side effects; I haven't had any side effects other than some drowsiness at first. I also use Zaditor eye drops (over the counter, generic is ketotifen) and Benadryl for sudden reactions.
I volunteer with cats. Some cats cause more of an allergic reaction than others. I haven't found a pattern -- not long hair vs short hair, nothing.
Haunted, have you and your girlfriend thought about fostering? You could give a cat some time away from the shelter (to raise her kittens to adoption age [8 weeks], to recover from surgery or illness, or sometimes cats are just stressed in shelters and need time out in a home) and tinker with some allergy meds and see how you feel. The foster cats would be temporary, so you could test-drive having a cat in your home before actually adopting one.