I think that Billy Miller's manager Marnie Sparers didn't exactly do a great job with her statement about Billy's manic depression. It leaves much more room for speculation than if she had said nothing at all about the circumstances.
The only fact that is known about Billy Miller's life in 2023 is that his father passed away on January 19th 2023 after a valiant battle with Alzheimer's disease. When Billy Miller sold his home in LA about two years ago, there was speculation that he would move near his parents. I remember when he was on the soaps, he supported events in Texas for his niece's sports team. His niece was born without her left hand due to a congenital abnormality. As a widow, his mother certainly needs a lot of support in the last few months. If Billy Miller really suffered from PSP and he saw in his father what a degenerative disease can do to a person, I can imagine that he didn't want to go through that and also didn't want to put his family through that, but the mention of manic depression gives the impression that it is something that he has been battling with for a long time.