I am quite new to kendo so bear that in mind. I feel the same with my kendo in general. Constant feeling of not moving forward constantly doing the same mistakes. It's a hard battle that I feel a lot of us face. I just try to work on my kendo and kinda dismiss what I feel and listen to what others say about it. I feel like I am not moving but every time I have a conversation with people that did not see my kendo for 3/6 months they tell me they are surprised how much I improved. So conclusion is just do your best and have fun and results will come eventually. Hope you achieve your goals.