LOL, welcome to the club, I get this ALL THE TIME, and it's been a frustrating uphill battle! We're the house band for a moderate sized saloon and a decent size stage, but I'm in the back corner right next to the drums, which is about the worst place any musician can be. Corners are bass magnifiers, by nature, and drums (to my left) are always going to be the loudest acoustic instrument on stage. Here's what I did, I have my own amp that I place right in the corner at a 45 degree angle upward, sitting on top of a plastic milk crate. The trick is to get your monitoring amp as close and focused at you as possible. I even built a simple wooden holder so I could lay it at a 45 degree angle up at my ear. This has cut down on the complaints greatly.
That often goes to shit during rehearsals or in other spaces though. The moment I turn it up just loud enough to hear myself over the drummer, everyone's giving me dirty looks. I'll get back to you when I've got it all figured out... probably never.
I also suspect keyboardists have a reputation like victorian children "Should be seen, not heard"... except for the "seen" part.