I resonate with many posts above. I nearly always have multiple projects. One pattern for me is that I tend to reach a point in a given project where my energy is flagging - perhaps I've figured out some problems, but now am not quite sure what to tackle next on it. So I switch over to a dormant project, and find that I have new energy for that one.
Another pattern is that I start a project, and then realize I could really use a certain tool that I've thought about building, so I switch over to build the tool. Then I realize that to make this tool, I really first need to make that tool, so I switch again ... and the situation cascades. I've been in this sort of cascade for the past year or so, gradually working on making a foundry to try my hand at some casting. But I decided that I could really use a slip roll to make the forms, so I switched over to that ... only to decide that it would really help to finally make the 2 x 72 belt grinder that I had thought about making for the past 15 years. So I switched over to that.
I finished the 2 x 72 (mostly - there are some tweaks and improvements I'd like to add, "some day"), and then fairly recently finished up the slip roll, so now I'm back on the foundry, and getting fairly close to finishing up the metal fabrication; then it will be time to insulate and decide on the details of the burner. No doubt there will be some other tool / jig / fixture that needs doing before I get completely done with the foundry ...