+1000 for what CaptJack says. Find a good mentor or instructor who can help you dial things in. It took me a few tries, but once I found someone who helped me do this to my tanks it was like night and day. Quote from that day: "Your trim is good but you are workin' way way way to hard for it". He dropped my tanks and boom! effortless. (this was after 2 decades of diving, thinking i mostly had things sussed out, proof that even old dogs can learn new tricks) For another person that day who was wearing very tail light 72's he dropped her tanks AND put a tail weight, and same result.
Balance your kit: drop your tanks (loosen your shoulder straps, tighten your crotch strap if you are in BP/W), flip your v-weight so it has more weight on the bottom if you are diving dubs, roll with a tail weight if you are diving tail light tanks, lower your weight belt with a harness or pouch near boot of tank. Once you do some of this, you won't feel like you are falling ass over teakettle if you stop moving and suddenly 'in trim' is far more comfortable and feet won't feel 'floaty' even with air in them
Think about the heavy parts of your kit and where they are placed on your back. That nice big chunk of brass (regulator first stage) right behind your head has negative buoyancy, so it is technically 'pushing your head down', The base of many (more modern) tanks are heavy, where your waist/weight system is placed will also make a a difference. the further up all that 'heavy' is on your upper body, the more leverage it has, and therefor the lighter your feet will feel.