Really, if you aren't getting good levels you need to turn up the headphone monitors. Or get more comfortable "performing" your singing.
Another trick is to face your real monitors, turn them down and sing with them.
A couple of things to check - make sure the bass roll off isn't on for the mic (don't know if it has it ..). Check to make sure the EQ is off on the mixer, then try adding some bass. ANd I hate to mention this, but are you singing into the front/top right spot of the mic? Just checking. Like I said, if you aren't getting volume you either aren't singing at a speaking voice volume or you've got something hooked up wrong. I doubt if the behringer has a great preamp, but it ought to work.
Finally, maybe your voice just doesn't work with the mic. It ought not be as bad as you say, but sometimes ...
@