Setting your own Eq while singing...

Before you can do anything in regards to making your own voice sound good through a foldback speaker, the sound of the foldback speaker needs to be investigated and adjusted if necessary and all before anything else is attempted.
 
I use a high quality reference track to tune FOH speaker systems.  I also use the same reference track to feed and setup foldback speakers .
 
One never really knows how a foldback speaker sounds on a quality music mix.  They tend to vary a bit more so than FOH speakers.  (I have heard some bad ones!)  Although I am hearing better quality foldback sound in general.  Thanks to pretty classy full range foldback monitors that sound killer nowadays.
 
I use two Steely Dan albums as my PA reference material.  The last two digital produced albums only.  'Two Against Nature' and 'Everything Must Go' Both are great.  But there are many great reference mixes to use. (Not all music is good for FOH and monitor alignment)
 
Set the CD mixer channel so there is no EQ anywhere and try sending that mix to the foldback speaker.  You need to stand in front of it now and listen to how it really sounds.  The foldback EQ should be flat at this point too.  The better the foldback speaker the nicer sounding that mix is.  Now the thing to do is to set that foldback EQ now to make up for peaks and troughs in the foldback sound.  Try and carefully use a few sliders on your graphic to improve the sound of that speaker.  Move one slider at a time only and keep the number you need to move down to a minimum as well eg two or three.  If you think there might be a peak somewhere try and figure where and pull that frequency down 3 or 4 dB only at first.  If a slider does not seem to have any effect then try the frequencies either side of it. If you feel there is a deficiency somewhere identify that too and add some slight boost there. Same 3 or 4 dB to start.
 
After a little foldback EQ the sound of that mix can be better now.  What you are after is a smooth well balanced frequency response of the whole mix from reasonable lows up through clean and clear mids and on up to nice smooth crisp top end too.  Not too much highs but just subtle amounts as they are in the actual mix of these tracks.
 
For foldback I generally add some extra roll off for lows down the bottom end  (this is important, it eases the low end in the monitor naturally bringing up mids and highs)  and some roll off for tops too after about 7 K or so (and steeply)  just to stop and high freq feedback squeal should it happen.  (be careful not to roll off too soon up high, as the music will sound dull, keep the response in up to say 7 K and then steep it down fast)
 
Now try sending your own voice into that well tuned speaker now and see how it sounds.  If you are sending post EQ then any mixer channel EQ's will effect the sound.  Or try sending the pre send as you normally would.  I have always found once the foldback speaker is tuned a bit then sending in even a pre EQ SM58 most often sounds very nice.  Try adding a hint of reverb to your foldback mix.  Not too long (reverb time) say 2.5 secs or so.  You should be lovin the sound of your voice by now.
 
Anything else after that is all about how you feel about your own voice and if and when you start to hear how your own voice sounds and then you should be really liking it by about now so you get into your ultimate vocal performances.  By then you should not be too worried about how any foldback speaker sounds.  You are immersed in the music of it all. If the foldback is sounding good then it only adds to the experience.