I would imagine that, following the death of their mother, William and Harry became a very tight unit - and that Harry has struggled with the changes that have happened as they both became adults, and when William married and had children. It’s not logical or emotionally mature, but maybe he felt abandoned, and that opened the scars left by his mother’s death. He is almost grieving for the relationship they used to have.
But while I can understand how he may be feeling, he is an adult - and an incredibly privileged one to boot - and he has the resources to get the therapy he needs to get over this. But it does look to me as if he is stuck in a bit of a victim mindset, and won’t be able to move on until he accepts that William has a right to have a happy family life with Kate, his children and the in-laws, and that life simply couldn’t stay the same as it was when they were teens, and that he himself has a life that many people would envy - a beautiful wife, two lovely children, a pampered life in a 10 bedroom mansion - and needs to grow up.