Sounds like you need a real gun belt.
The holster is only part of the solution, but the main way to insure your gun is tight up against your body is to use a real gun belt. More than anything else, the gun belt will make it easy to carry, a heavy gun seems light, a light gun seems weightless.
This says it all, I weigh 140lbs and carry a Sig 226. Since I bought a real belt no more floppy gun, saggy pants or a sore back.