I've been dedicated for 6 years and it took me 5 to find out my Guardian, too. At first, years ago, I tried following that method many are familiar with, which consists of narrowing down the list of Demons on the JoS to one or two which you find to have the most affinities with, in regards to their attributes, what they do, their gender, elements, etc. I narrowed down my options to Agares and Vapula, but at the time I didn't try contacting any of them to confirm it.
Last year, I thought of asking Satan to give me a sign of who my Guardian is. The next day (and then for several days afterwards), I kept seeing images of tigers everywhere, coincidentally. The tiger is Agares' sacred animal. Long story short, several weeks afterwards, I decided to talk to Agares in my mind and ask her directly for a sign of confirmation that she is indeed my Guardian. That sign came one hour later in the form of yet another coincidence, and by then there was no doubt left about it.
I'd like to further elaborate about it, but I'm in a hurry as I write this. But I will soon be posting something both here and in the forums where this will also be covered.
For now, the point being is, I'd suggest you to do the same. Ask Satan for a sign of who your Guardian is, and keep an eye open for small coincidences. The Gods will often communicate with us in that way.
Hail Agares!
Hail Satan!