Single word for "refusing to move to next activity unless present one is completed."
A perfectionist is defined as follows:
a person who refuses to accept any standard short of perfection.
But I am looking for a noun which describes a person who refuses to move forward in an activity or thought unless the present one is sorted out. Kind of like having a mental block to move ahead, but by choice.
Solution 1:
It's not listed in the major dictionaries, but Wiktionary mentions completionist:
One who insists on completion.
I wouldn't count on your audience knowing exactly what it means, so you might have to explain it first, but you probably wouldn't get any closer with a single word.
Solution 2:
I would use single-minded (Merriam)
: having one driving purpose or resolve : DETERMINED, DEDICATED
or unitasker (wiktionary)
A person who does a single thing at a time.
Solution 3:
Serialist could have something of the meaning you want. The problem is that it has some specific meanings in other contexts, and doesn't seem to have been used in relation to the completion of tasks. The closest meaning in the OED is meaning #4, but it relates more to learning rather than the completion of tasks.
A person who writes stories, novels, etc., for publication in serial form.
Philosophy. A believer in or advocate of a theory based on the analysis of the self as a series or succession of states or events. rare.
Music. A composer, advocate, or admirer of serial music or composition.
Psychology. A person who tends to acquire knowledge about something by consideration of a series or sequence of items, facts, etc., in turn.
Examples given for meaning 4 are:
"Serialists learn, remember and recapitulate a body of information in small, well-defined and sequentially-ordered segments."
"Operation learning is the style of those who are routine serialists; comprehension learning that of routine holists."
More broadly, you could say that such a person is "stubbornly serial" or "doggedly serial" in completing their tasks, though you'd probably have to add a subordinate clause explaining what you mean.
Solution 4:
Single-threaded (a term borrowed from computer science)
In computer programming, single-threading is the processing of one command at a time. The opposite of single-threading is multithreading. Thread (computing) - Wikipedia https://en.wikipedia.org/wiki/Thread_(computing)
"He had a single-threaded mind"