Another view is does your saddle fit you? Everyone’s pelvis is different. I need a narrow twist and a slight rise in my saddle for my sloped pelvis. Otherwise I get tipped forward. I have had the same problem with my dressage saddles, and a saddle fitter pointed this out to me. I am solid in my dressage saddle, even for a gallop, hills or whatever. My new wade saddle, while it fits my horse, doesn’t fit me. I have fought my position since I got it last October. It tips me forward so I constantly am trying to sit back. I found a little barrel saddle that is perfect for my pelvis and my leg falls right into place and still fits my horse almost most as well as the wade.
If your saddle is not right for your pelvis, it can tip you forward and your leg may be too far back and that’s why the trainer is trying to get you to bring your leg forward. Or, he is one of those that has you shove your legs forward and sit too far back on your butt.
So, really pay attention where you are sitting in your saddle. Is your pelvic floor square on your saddle seat, do you have any space between your crotch and the saddle? I can stick my hand between me and the saddle if it doesn’t have enough rise for me. Alternately, if you are bumping yourself and it isn’t comfortable, a flatter saddle may fit better. My sis in law needs a flatter seat with the narrow twist, but our horses need the same wide tree. Tricky sometimes to get the right fit for both horse and rider.
You haven’t posted any pictures, it would be easier to see how you are sitting with pictures.
Good luck.