On that note, I don't understand the comment on page one about my friend's suggestion of a different BCD - could you elaborate please?
When you're underwater, & correctly weighted, you'll have very little gas in the wing. Because the bubble want's to rise, when you're horizontal, the gas will be in the outside edges of the wing, & the wing will be wrapped to some degree around the tank. The tank is cradled & so supported by the wing when using a back inflate or backplate & wing. Even with a jacket style BCD, most of the gas will be in your back region & will support the tank to some degree.
One more thing about wings cradling tanks. The longer the wing, as a proportion of the tank, the better the tank is supported. Back inflate, jackets, & some wings are quite short compared to an aluminium 80 or a steel 100, etc. The weight in this over hanging tail on some tanks, may work as a lever to pull you off the horizontal if you don't counter the force with good weight distribution.
When you're vertical, the gas will shift into the top arch. It'll be up around you upper back, shoulders, neck region. The levers & fulcrum have moved dramatically. The tank is no longer cradled, & is not supported by the wing. This is more or less the case for any style of BCD
That's not to say I'm into this "you must be horizontal/use a backplate & wing, or you're a poor diver/gonna die" stuff. I'v been known to ascend head up along the side of wrecks at a 30
o backward cant to the horizontal.
I'd be looking at proper weighting & its distribution, more input from instructor, & of course it all becomes much easier as you gain more experience.