I used to think I could multitask well, but in the end, I had the same difficulties that you are describing. Actually, it got so bad for me that I literally blew up. Eventually, it was quite well-known that I had a mental illness. I didn't willingly disclose it. It showed itself against my will. That fact did allow me to receive some accommodations. However, I understand people not wanting to have mental illness known.

My old job did allow me to pick and choose what I worked on, to some degree. Some of the time, I was expecting myself to do more than others were expecting. You might ask if that might apply to you. If not, another idea might be to run with a portion of your job in a way that shows extra creativity and initiative. Having such an plan that would significantly benefit your employer can sometimes make them quite willing to let you hyperfocus on one or two things. Present the plan, with advantages list, and see what they say.

Of course, I don't know what you do, so am unsure if my advice is applicable/helpful. I will say that one need not fully disclose their mental illness to ask for some time to focus in one (or at least a couple) directions, rather than several. Most all people need some period of a "break". Any kind and reasonable employer knows that its best to keep employees healthy enough not to totally breakdown. To reiterate, sometimes it is the employee themselves that brings on the crash.