i have weight in my heels and they are down and i try to relax my hips and just let them do whats natural, but i rise a little and i am rocking my hips. i am much looser, it just seems like the rhythm of the gait is just to big to sit to BUT when galloping and cantering up and down slight hills, i can sit with the rhythm perfectly.
I just got back from galloping today - so hopefully I can help while I still remember my ride in mostly full detail ;) .
I also have a hard time sitting a gallop - but I can to a lope just fine. One thing I have heard and read, is that if you barely lean forward so you have just a little more weight in the stirrups and your bum is about a half inch off the saddle (kind of like you are riding in your stirrups, but not really) then it can help.
today I was bouncing like crazy, but once I got up in the saddle a little bit it really helped. And if you are having troubles kind of staying that way to begin with, then I found that pushing up on the cantle a little bit helps as well.
my right leg swings back and fourth more than i would like it to. i still keep weight in my heels, but it still does it anyway and i have no clue why.
I found that when I gallop, if I keep my feet mostly perfectly level with the ground and just kind of even the weight out - it helps with the swinging in my legs - mine do it too.
But just keep them level with the ground and just put a little bit more weight in there than you usually would.
If you are having troubles sitting a lope as well, then I found that if you hold on to the saddle horn, but push back so you are kind of sitting on your pockets in the cantle, then it helps to keep in the seat and ride it a little better.
I hope this helps you because it really did help me today when I was riding.