First, adding weight is likely to exaggerate the problem - more unnecessary weight = more air in your BCD. Weight low on the torso, Air high on the torso.... more deviation from the horizontal. Seek to confirm your optimal weighting (
reduce?) as the first step. You only need enough weight to maintain your safety stop with a near-empty tank and an empty BCD (
see buoyancy article below for how to conduct proper weight checks).
Second, think about the distribution of positive/negative items on your body when you dive. Heavy fins + long lever (
the legs) can drag the feet down. Thicker wetsuit booties and lighter (
less negative) fins can reduce the problem. Use some of your weighting for optimized trim - move it off your weight belt and distribute to help hold you horizontal. There's many threads on that here.
Lastly, body positioning is important. Try holding your legs bent at 90 degrees from the knee (
upwards) - that reduces the lever effect. Think about your hand/arm positioning - it has an effect also. Same for your head (a heavy thing!), is it help up, or looking downwards... that has an effect also. Keep your back slightly arched (similar to a parachute stable position) - if you let your hips bend, your knees will drop and your trim will revert to more vertical.
Explained in more detail:
Buoyancy Masterclass Series
10 Tips for a Better Horizontal Hover