I went to the field to have more space. I wanted to work on flying changes. However, things didn't go well. He was very good from right to left and not so good from left to right. He was late behind, very late. I felt like he didn't have enough balance and couldn't jump through with his hind end. I tried to work on that and he started to get really upset until I couldn't continue. I knew the issue was balance. But I couldn't find the piece of a puzzle I was missing. I stopped my work on flying changes let him walk and calm down. It took about five minutes. After that I started doing canter-walk transitions just going across the field. I was asking him to be sharp, collected and respond instantly. After a few transitions back and forth it hit me. I was off balance. I was throwing my right hip forward and up and that made him fall to the right. During changes I was trying to fix it with the half-halts on the right rein which of course didn't help because it didn't correct my unbalanced seat. I didn't come back to flying changes. I need to work on my balance first. I finished with excellent collected canter, best I have ever felt him and very clean transitions.