Hi, FiddleNich. for a start try to cut the feedback from a specific frequency band, or all the low freq the violin doesn't produce. You can hook up a 31 band single channel Equilizer thru the channel insert plug if you have one on the PA mixer. Route 100% of your mic channel thru the effects-insert loop, not just part of it like you would with a reverb.) If your lucky it is another instruments sound coming from the monitor at 200 or 300 Hz that cause the feedback and EQ-ing that out solves the problem for a high-pitch instrument like a violin. Just start playing your violin low notes and gradually cut the low frequencies completely and see where you notice an effect on your sound. Stop there and see if you still have feedback with the full group playing. If none of that helps, get the feedback started and selectively cut EQ frequencies one at a time raising them back to 0/unity gain at each one. Do this until you find where the feedback frequency(s) is, see if you can live with your violin sound with the EQ set to cut there. (the more eq bands the finer this can selectively cut that particular frequency range, 31 bands is probably min for this purpose.) All that said, the feedback may change frequencies from room to room and from room temperature to temperature. I've set up in daytime temp, sound check was fine and show at night had feedback because the cooler air is thicker, or the room full of people makes it warmer and thinner.
The only perfect solution would be an in-ear monitor for you, while you stay away from or in front of the band's floor wedges. Or get them for the whole band and dump the monitors. The other plus is that on a loud stage, in ear monitors in both ears can be real hearing savers. I keep that in mind because I already have a man-made replacement for my left stapes bone (the wishbone shape in the inner ear.) It works better than my damaged one, but no where near as good as the one I was born with.
Beyond in-ears, you can work on the angles of your mic versus the angle the monitors face up at you. You probably get no feedback when the violin body completely blocks the floor monitor from the face of the mic. It starts when you tip the violin towards vertical, towards the monitor front. Floor wedges when you are right over them and tip the violin to near vertical are going to be well within the under -3db rejection of a cardioid shape pattern, IF the mic face is straight down 0 degrees at the strings. If mic face is at 90 degrees to the strings it will be pointing the mic butt end (that the cable comes out of) at the monitor, which is what you'd desire but 90 degrees to the strings is too much off angle. With a cardioid pattern you have 60 degrees around 0 on the front face with perfect reception of sound, and 60 degrees at the BACK around 180 degrees of that mic with -15 dB rejection in this case. Thats 5 times less receptive in that back 60 degrees! Try to get the mic face at the edge of its 30 degrees off axis to the strings, while the monitor is in the opposite 30 degrees segment (off 180 degrees/the butt end ie. 210 degrees) . Sort of like a teeter totter line drawn through the long axis of the mic, a flat teeter totter is a straight thru line is 0 to 180 degees, and one that diverges 30 degrees to 210 degrees pointing at the monitor on one end and the violin surface and strings at the mic face end of the line.
Another fix is to get the monitors closer to your ears, so they can be turned down and quiet the whole stage, also possibly make that 30 to 210 degree line achievable. Peavey makes a 3 driver box call the Impulse II that mounts on a microphone stand - much closer to the ears. And your current monitor power amp can probably run 8 of them easily so that gets them closer to each band members ears. They are 16 ohm resistance so 4 can run off one power amp channel to present a 4 ohm load - and they have attenuation POTS so they can be individually turned down. Start with them all the way up, set the amp feed as loud as the deaf-ist member wants, then everyone else can turn their own down to what they'd like. I suppose it would be half the cost of in-ear monitors, the boxes are about $130 each. I've bought about 10 off eb-- averaging $60.
I've seen symphonies using clear plexiglass to protect the hearing of the front row players from the horns in the row behind them, such as is done around drum kits in live recording situations and churches. Maybe it could be fashioned into something that blocks direct line monitor to the mic, but not to your ears. Probably only work in one posture like a symphony musician tho.
Sometimes the stage is loud because one or more guitar-bass-keyboard speaker-amp is/are to loud and monitors have to be turned up to match that. Get these guitar amps up off the floor 2 foot or more on risers/stands so 1. the instrument's player's ears hear how loud it is, not just their ankles, maybe turn it down some, and 2. the crowd also hears it better! Could then use the plexiglass screens in a tall, narrow panel behind just you? The only thing that really sounds better on the floor through floor coupling is a subwoofer, and you should never have one behind you on stage or deafness is arriving very soon.
Now I'm only doing brass based swing band music, only the vocal and e-piano go thru the monitors, the ones I mention above on mic stands. When I did church worship groups, they always wanted more and more monitors from floor wedges. I could turn off the front of the house speakers and hardly anyone in the first 10 rows noticed a volume change, because all they heard in that area was the stage monitors that faced away from the room! Too much stage monitor sound volume plays havoc with musicality and makes the front of the house sound muddy because the PA speakers are competing with the stage monitors that project out the back of the monitor box (all bass) the omni-directional component of all low bass sounds and reflection off the back wall-ceiling corner that the wedges aim at (primarily bass frequencies get reflected, out of phase and out of timing). Vocals and mid frequencies get lost in the monitor noise. Try to achieve a quieter stage. Quieter stages are more musical stages, are you making music?