12 lbs with no exposure suit at all seems like an awful lot of lead. Beyond that trim is very dependent on how you hold your body underwater. Are your fins horizontal? Legs straight vs knees bent fins vertical vs knees bent fins horizontal are all drastically different. If your fins aren’t horizontal you loose the up/down reaction surface at the furthest point from your center of gravity, you’re going to feel a bit out of control. As a friend of mine would say, get your form right and STOP finning, see what happens.
With all that said, give yourself some time to get comfortable in the water. See if the instructor will do another weight check with you, extra weight you don’t need makes everything harder. 500 psi in your tank, you should be able to sink when you exhale and fully deflate the bcd.