My answer is simple. Take the meaning of each word. Ask for longer spaces between words if your copying is carried over to the word coming next. Our recent memory buffer is rather small. Unless we understand what the word/sentence means, we could hardly go on head copying.
On the other hand, in addition to the advices from less experienced hams such as just encouraging to go on learning etc, there is the advices from those firmly believing in writing down the codes literally. Some of them seem to be or used to be professional R/O. For them, CW must be a mode to copy completely on letter basis. But for us, amateur radio CW operators, it is to communicate with others. As I have reiterated here, writing down requires excessive function of writing, which is often disturbing to take the meaning of the message. When writing down, we should concentrate on motor function of the arm but not on taking the meaning. Of course, most R/O are capable of reading CW by head even without writing down the message. Their capability of head copying has come not from training writing or typewriting but from reading the message on word/sentence basis.
There are a lot of "CW operators" who have been educated with writing the message on letter basis. When they are trained that way, they won't be able to convert to head copying later. Writing itself should be the aim for them. In such a case, it is often difficult even for the native operators to change from writing copy to head copy. They could be psychologically dependent on writing process.
Anyway, this thesis, I am quite sure of its rightness from own experience and observation of a lot of CW learners in the past. But there are still a lot of arguments about it. Some people, apparently not able to do head copying, insist their own way of writing training. It is as if we learned music instrument in the internet. We should look for a reliable elmer as a reliable teacher in music.