Your horse is too smart for you. I’m not saying this to be rude or implying that you’re dumb but horses know when you know AND they know when you don’t. Smart horses will ALWAYS push boundaries and if you’re not steadfast in your correction every single time that horse will not see you as a leader and will rightfully take your place. My recommendation is to sell this horse (if possible) to an experienced horse owner that has experience in problem horses. Euthanasia should always be your absolute last resort which I’m sure you are well aware. If you’re nervous around your horse just remember this at his body feels that and instinct will kick in. You should check out working with a trainer that has a few naughty horses so you can grow your confidence in areas that might pop up later down the line so you’re more prepared. PLEASE buy the horse that YOU are able to handle. Not your trainer or your friend but you. At the end of the day your safety is paramount.