Yeah, now that I looked at the harness, Bob has a good point. Try adding a new shoulder stop-rope to the harness, and see if that helps. This new rope goes from the front of the right shoulder, through the carabiner, to the front of the left shoulder. It should stop the forward rotation of your body when you are comfortably head-level.
If there is no good way to attach a line at the front of the shoulder strap now, you can try it out temporarily, with just a thin rope.
To add a Nylon rope where there is no pick-up loop now, here's how:
Put a piece of wood inside the harness, where you want to make a hole for your shoulder rope. Gently, poke a pointy (#1) Phillips screwdriver from outside the harness, into the wood. Twist and wiggle the screwdriver point against the wood, patiently, until it penetrates the fabric. You are separating the fibers of the material, NOT
cutting a hole. When the screwdriver goes through, trade the screwdriver for a pencil, and continue to enlarge the hole until you can pass a thin rope through the hole. Use a hot soldering iron or hot blade to cut and melt the ends of the thin rope, and make them pointy. Put the thin rope through the hole, and tie the rope back onto itself, making a loop. Do the same process, for the other shoulder strap. Do a long hang-test in the shop with your new shoulder stop-rope, and you can even fly it like this. When you are done testing the new shoulder pick-up rope, you can just remove the new rope from the harness, and wiggle the fabric between your hands, to make the hole almost disappear.
If you like the results with this new rope, any decent harness maker or parachute rigger can sew new webbing loops at the shoulders, for the new shoulder rope.
If you can break the sewing thread with your bare hands, without cutting your hands, that stuff is NOT