I used to have to juggle a lot of different clients and projects in parallel in the web agency I owned, since i did sales, client partner, support, general development (no, it wasnt just me, we were 20, but the buck stopped with me so i dipped a toe in a lot).
I discovered that I get so into the moment that I do not stop to record the swap from task to task, the interruptions, phone calls, or the meetings
A tool that helped me was one that records the details of the title bar and open files of the apps I have open, how much focus time and %of CPU they had. It also pops up a little window if I am been inactive for 5 minutes asking "what did you do? break?meal?phone call? meeting? for which project?" in an easy to deal with dialogue (5 seconds max).
With that tool I can review at the end of the day and tell it which project each line was related to. I can make the assignment sticky (i.e. "file xy in word is project z" "site T in firefox is project R" "donationcoder.com is break" "email thread topic Q is .." etc) so after a week the daily review is quick.
It is the only way I ever found out to actually get an overview, all the systems that rely on doing it while it happens failed myserably. I am too much in all the things I juggle to have that meta "record task swap" moment
There's a utility here, is it called titlelog, that does a simple version of this, without all the mapping and reporting (which you could do in excel or...)