I can only speak from my own experience, and YMMV. I found that two things changed my comfort level the most: 1) taking a sidemount course from an instructor that gave me the attention and had the expertise to get my gear, trim, and buoyancy 100% under control (well, buoyancy was about at 80%). 2) taking classes that increased my task loading. This will seem counterintuitive to many, and it's not for everyone, but I find that I will overthink and over-tinker with by body and gear to get everything perfect. But, when I have new tasks, new skills to work on, the more basic skills sort of fall in line and become mindless and natural. I am NOT advocating moving too fast into more advanced skills, but classes like the GUE fundamentals class, or a specialty class that covers one topic in depth (i.e., NOT AOW) from a really good instructor can take you to a new level.