Doing one thing by definition excludes the countless other things that you could’ve done in that time. Thus, joy is felt only when you feel within you that the thing you are doing right now is more important than the other things possible.
I recently learnt from the excellent book 4000 weeks by Oliver Burkeman that the word ‘Decide’ comes from the latin word decidere, which literally means to CUT - OFF. To decide then is to mean not to choose something but to cut everything else off.
So when you Cut Off, make a clean cut best you can. So that the only thing that remains is the one important in the moment. There will be countless other things equally important but you have to choose and stand by the one you have cut off.