It's okay; everybody carries more weight than they need when they're new. My first tropical trip, I took 14 lbs into the water with me. Now I dive with six -- same wetsuit.
There are a lot of reasons for difficulties descending with new divers, and most of them aren't being underweighted. First off, there is the coordination of your breath. One of the things Bob taught me was that, when you start to empty your BC on the surface, start to INHALE. Everybody thinks they should EXHALE at that point, but if you do, when your head goes underwater, you're ready to inhale, and that stops your descent. If you inhale on the surface, then, as your head goes underwater, you do a forced exhalation, empty your lungs completely, and you'll continue down.
Secondly, most people are finning as they move down into the water, and with your fins beneath you, you are driven back toward the surface. There are a bunch of reasons why you fin, but one of the most common is that you're tending to go over on your back, and you're kicking to stop that. Happily, the same solution fixes both -- Cross your ankles with your knees bent. Crossing your ankles keeps you from finning, and bending your knees puts the blades of your fins behind you where they will catch the water as you sink, therefore pushing your feet up and your chest down, putting you into a nice horizontal position in which to descend.
Yes, you can trap air in the wing and in the suit, and you have to learn to vent both, but ineffective breathing and kicking are much bigger reasons for not sinking.
Good luck getting it sorted out -- When you learn to descend properly, you'll find you can shed weight like crazy.
