As far as feedback goes, there's not a lot you can do in an enclosed space except either turn the speakers away or get yourself a more directional mic.
If you have access to an EQ, you might be able to dial out the frequency that keeps feeding back too.

For earplugs, you'd need to get used to using them... just keep using them until you learn how to compensate. Either that or turn down the monitors.

You're right to be concerned about ear damage btw. I had a major scare a coupla years ago when I let loose with a really loud guitar solo. I damaged my hearing so bad, I thought I'd screwed it up permanently. (if ya wanna majorly freak yourself out, try going around for a few days not being able to hear simple stuff like the sound of your own footsteps or the sound of your fingers rubbing together right next to your ear. It scares the crap outa ya)