Huck,
The only absolute certain cure I know of is amputation! Assuming you'd prefer not go that route, try Ivy Stat. It's available at drug stores. Considering the duration of your infection, I'd have to agree with KSDan, you must be reinfecting. I get some EVERY year. It usually takes about a week to blister up from when you got it, a week of intense itching, and a week of it going away. Sounds like to need to wash EVERYTHING that was anywhere near that ivy. You must have it on something that you touch a lot; your bow perhaps?