Adult onset stutterer myself. In a very similar boat as you. Truly a frustrating mystery. I think the lack of answers is worse than the actual stuttering for me. From what little I’ve been able to dig up it does seem to be a symptom of chronic severe depression. Mine started at what was prob the most stressful point in my life, and does seem to get worse when I’m under a lot of stress. Recently I’ve been trying to learn about autism, and a condition called autistic burnout. I’m pretty sure I have an undiagnosed ASD, and I wonder if it’s connected to the onset stutter. I’ve read it’s pretty common for people with ASDs to stutter so seems like it could be connected. Good luck, and hope this helped a little. If anything just know you’re not alone with this horrible condition.