I'm sorry that sounds tough.. I don't have germ anxiety but I think SSRI's can help you. I have a good friend who has anxiety and OCD (not germ-related but she gets irrationally worried about having various health problems when she's not on prozac) and they have helped her a lot with her irrational health anxiety.

I don't have relatable experience or anything that will be helpful other than explaining the logic that touching a door knob will not make you sick and that excessive stress does lower your immune system... but you know that... but I think that if you can see a psych and get on meds it could really help you if you haven't already. Either way good luck!