We have a yamaha powered mixer at 350 watts a side going into 2 2x15 cabs, yeah plenty of everything.
+1 to the long-term fix being voice lessons to help your singer project. In the meantime, though, you need help getting through gigs.
We're guessing about your venues, monitor system, band mix, etc. We're also guessing about the functional problem. Is it that you can't get enough gain for her to be heard in the FOH, for her to hear herself in the monitors, or for the rest of you to hear her in the monitors?
If you mute the monitor system to prevent the stage wedges from feeding back, is your singer's mic still feeding back in the house mains when her channel has enough gain to put her over the instrument bed?
If so, you might need to pull other instruments and voices down in (or out of) the FOH mix. You can also play with mics that have tighter response patterns and that fit her voice better. In the most extreme cases, you'd have to pull the instruments out of the mains *and* bring them down on stage. Until your singer learns to project, you'd be "that quiet band."
Or is the problem originating with stage monitors? If so, start by ringing these out to address any serious frequency spikes. If her mic is feeding back in well tuned monitors, you can lower stage volume, move her to in-ear monitors, or both. If she doesn't fly all over the stage, I'd start with a simple wired in-ear path of mixer aux out --> limiter --> headphone amp --> ear buds.