I have the same issue and what makes it hard is that people always just say “put your shoulders back”. What I find works much better is to focus on the ear to shoulder segment of the straight line from ear to heel. I also find that getting my shoulders taped really helps as it is a constant reminder to have your shoulders back. If a few months go by and you still can’t keep your shoulders back, there is a training side called a shoulders back by equifit. It works well but will really limit your mobility because it holds everything so securely in place. There are also some other shoulder braces you could look into that are less severe, but this one is specifically tailored for riding.