First off, what gear configuration are you using, and what exposure protection? If you are diving doubles, being feet-heavy is almost unheard of. People almost ALWAYS rear up out of the horizontal when they switch to doubles, but the reason is because the manifold and regs makes you head-heavy, so you rear up to shorten the lever arm that those things have to work on pushing you down. The solution to a head-up position in doubles is almost always to move as much weight down as possible . . . but that assumes good posture to begin with.
Posture is amazingly powerful in balancing gear. Posture is the reason that people like Bob Sherwood and Andrew Georgitsis can look beautiful and stable in almost any configuration. They are absolutely flat from shoulder to knees, and use their arms and their lower legs to fine tune their balance. Of course, this can't account for gross imbalances in equipment, but it can do an amazing amount. A lot of us struggled with doubles when we started, and find, a hundred dives or so later, that we can't remember why it was so hard.
Gross imbalances, like trying to dive doubled LP95s, HAVE to be amended with weight. When I dive double 80s in Mexico, I put a 7 pound half v-weight BELOW my bottom bolt -- that both weights me and balances me perfectly.
The test is to get in the water with a buddy. Get as flat as you possibly can; get your buddy to give you feedback on whether you are dropping your knees or your head. (Video is the best, but you can communicate a lot with hand signals.) Get horizontal -- buddy can give you the okay when you are there. Then STOP MOVING. See where you tilt. If you tilt head-down, you need to move weight down. I would be willing to bet good money that that is what you find.
