Marshall
This is a subject I am interested in because, for some strange reason that sssss sound has started creeping into my own vocal recordings. Never been an issue before, but for whatever reason it seems to be now.
Rather than attempting to remedy it with processors, I'd be inclined to approach this as a tracking problem. Unless you've somehow developed a new whistle between your teeth, it's most likely an issue with the acoustics in your vocal space, your miking technique, or even a problem with the microphone itself.
The fix could be as simple as adding a pop filter, singing a little off-axis, rotating the microphone a few degrees, moving the mic further from a wall/window, or draping a packing blanket nearby. You'll need to try some experiments.
There was a time when I used to spend hours fixing vocals in the mix. Of all the various remedial tactics I've employed, at least some of them must have worked because it's been ages since I've had to fix essiness on a vocal. The last time, it turned out to be a broken microphone.
(BTW, one of the most effective changes I made was switching from an expensive condenser to a cheap dynamic mic for recording my own vocals. Maybe if I had a better voice I might need the greater accuracy, but the good old '58 fits me like a glove. I just have to use some self-control and avoid eating the mic.)