I know the feeling. It can be tough. I think I had 900 unread emails in my Gmail Inbox when I first started GTD. These days (three years later), my inbox is generally empty once or twice a day. I say this to give you hope.
From your comments, I think the really revealing thing you say is that the only source of relief you get is when you can put things in the 'trash'. I would honestly spend sometime meditating on why this is the case.
My guess is that the relief comes from having made a final, and trust-worthy decision about these emails.
My guess is that the problem with the rest of your processing is that you aren't really being honest with yourself: for example, that folder from 2009, I think it's worth saying that it is currently *in the trash (or perhaps a reference folder)* - you haven't looked at it for three years. The question isn't "What am I going to do with it?" The question is "Do I want to take this out of the trash/reference, or am I going to be 100% happy about leaving it there?"
I think that some of the classic self-help literature is useful here: the most powerful word for getting control of your life is the word "No". I get the feeling that you aren't really genuinely saying "No" to lots of things that you know in your heart-of-hearts you aren't going to get to. This creates psychological tension, and means you aren't getting relief from GTD.
My suggestion - and it's just a guess - is that you probably would benefit from having fewer categories you are putting stuff into: it's either (1) trash (delete), (2) reference (archive/put in a folder), (3) next action (and I'd suggest this is stuff you are going to get done in the next 7-10 days, including any reading), and (4) someday/maybe (a set of stuff you commit to reviewing weekly, but which you have for the moment decided to NOT do).
You can have sub-folders of these categories, but I think the thing that will give you real relief is if you can force yourself to decide which of these four categories each email fits into.
Clearly what is probably going to need to happen is that a lot of stuff (99%) is going to go into trash, reference, and someday/maybe - you are going to say 'No' to 99% of stuff. You just have to. It's a fact.
I had a similar experience to you with my physical filing - I had about 20 boxes of files I'd collected over 15 years - and they were completely unorganised. I went through and processed them and put probably 70% in a reference filing system, and about 29% in the trash. In the three years since that filing I've probably consulted those actual files about 3 or 4 times, but boy has my conscience been clear!