So I've got my instruments mixed to where they sound pretty friggin' sweet and while working on getting the vocals in I fear I've lost my way.  I've gotten it to sound pretty good using several different EQ curves, but at this point my ears are shot and I have yet to hit the sweet spot.  What sort of specific information can I offer up to get some guidance?

My best advice is stop right now and take a rest. Put it off a week and more if it feels right. Don't mix when you are not at your best. Unless you have a commercial deadline there is no reason to rush it.

It seems by your description that you are nearly done anyway so a little time taken off wont hurt anything.   

Sorry, have you actually got tinnitus or just one session ear fatigue?  I have tinnitus and it poses special (permanent) problems.

I think John is probably right, good to take a break..
A friend of me managed years ago to get nearly rid of the tinitus
by doing autogenic training 1 hour per day, but this is sure no news for you?

I too have been suffering from tinnitus for about 7 years now.

There are good times & bad times of the day - lunchtime is always bad, so is late at night, but I can just about blot it out, but it's not easy.

See - just the mention of the word and now I can hear it plain as day - haven't noticed it until I started reading this thread.

Here is some mixing tips from a mix expert!

Do you want vocals to be in your mix at the end of the day? If so pull all the instruments down and start with the vocal sound. 

Tip: If you want to create a mix that has instruments and vocals the mistake many make is getting all the instruments happening and then at the end bring the vocals in.

Start with the vocals. The vocal sound is much easier to get right when you are only hearing them. They will be easier to EQ. The vocal reverb will be easier to setup too. Any tinnitus issues will be easier to handle because you are only dealing with the vocals at this point. Easier for the ear to manage.

Now if you have got some great instrument sounds happening then good. Start bringing them back in. Make sure the vocal stays present and they are the main thing you are hearing. If the instruments don't sound great once you are hearing them and the vocals then alter the instrument sounds to suit and keep the vocal sound intact. Not the other way round. 

Switch to a small mono speaker at low volume to fine tune the balance between vocals and instruments. You will end up with a fine mix at the end of the day.

If there are vocals present they reign supreme. This approach does depend on a stellar vocal performances though. 

Have you tested several EQ-curves which all sound OK, or are you using several EQ-curves on top of each other? If the latter, then it sounds there could be something wrong with the original take (the microphone, acoustics or something). Or maybe you're not used to hearing your own voice and you're trying to achieve something that isn't there? IMO a decent vocal take "always" needs some EQ'ing, but usually not very heavy, the only "drastic" detail being the high pass.

I'm only speculating, of course, as I don't know any details.  There's no way of giving EQ-advice without hearing what you hear. You can do high pass and some boosting and/or cutting with one EQ-curve. If you're putting several EQ's VSTs in a row, that may cause undesirable changes in the sound, as most EQs have some special characteristics of sound of their own, which don't necessarily work together when stacked.

Do you have any of the most common practices in use: Delay+reverb, doubling (artificial or real), chorus...?

I do my best mixes when I (make a copy and) pull everything down to the bottom and do the mix over. This includes FX.
It seems hard to consider after taking all that time to get it right, but it goes so much faster the second and third time around.
And faster also seems to make it better. There is a confidence factor that seems to help.
As per posts above, I like to start with just the most important elements, usually vox and low frequencies rhythms (kick and bass) and fit the rest under quickly.

Dont know if you have time or money for this, but on my current project, I am getting all the mixes as good as my powers will allow, then listening to them in various settings, and then bringing another engineer I trust to make corrections.  He is a good guy who will not charge me unresaonably.

Obviously, this is overkill for a demo, but I don't want to go to market with something I don't feel confident about (which is funny since I hear some crazy stuff out there for sale).

Great info from all, thank you. Yes I have true tinnitus, and yes I'm only running one eq at a time (not compounding).
The two biggest things (other than jacked-up ears) are that I'm a guitarist by nature and suffer from hate-my-own-recorded-voice-itus. Only doing it all myself because my partner in crime is too busy raising three kids and maintaining a house, if I can't get him to sing then hopefully I can borrow his ears long enough to get an objective opinion.  Figures I wouldn't be this motivated to do all the work to make music until after my ear went to hell.
Thanks again, I'll rest when I die :) (okay not really but I've got to at least finish this one song soon or I'm going to go crazy)