I suggest you keep going at your original speed because I think over time your concentration ability will increase. As you become more skilled you will relax a bit and concentration will not be so much of a struggle. .
As a matter of fact, on this website is adviced 20/10
For the average student the progress is in that case very slow and as statistics show he/she will give up and quit.
Alternative is 20/4 In that case you can do the course much faster, hence it is less frustrating and after finishing you are able to make some QRS QSO's. Time to repeat the course with 20/8 and doing your qso's wide spaced at 20 or regular at 8 wpm. When finished 20/15 and finally 20/20.
With 20 wpm regular spaced, you are not handicapped in any way, it was the regular speed of coastal stations sending their daily news bulletins from shore to ships.
A ham able to copy 20 wpm is OK, only freaks, just as I am, try to reach higher levels. Stupid behavior, I know, because I can better write a book or designing, constructing and publishing ham-equipment, because the moment I pass away all Morse proficiency is gone.
However, I hate to ask for QRS