How is it possible to feel so stressed out with what seems like so few obligations?

I don’t feel like I have a mountain of task to complete, but the stress has already started my morning with a headache.

I can’t think of all the things I have to do without a headache starting. This morning, it hit me like a sucker punch.

It’s not that I feel like I have too many things to do. It’s that I just feel like I don’t have the bandwidth to finish — or even begin — most of the tasks. I just don’t get it. I don’t have a job. I don’t have a boss who’s persistently writing me about deadlines and things like that.
How is it possible to feel so stressed out with what seems like so few obligations? What am I missing here?