Just going off my own experience, I know that having regular lessons for myself keeps me in the right direction. I don't think that I am anywhere near experienced enough to assume that my work is 100% correct at all times without some guidance. In fact, there are not many around the world who could claim that, if any. I've ridden a lot of horses, a lot of schoolmasters etc. and I do think that I have a good feel on a horse. But I'm still not confident that I can train a horse entirely on my own, as a competition prospect, with no coaching guidance.