The traditional measurement (FCC Code exams in the past) was one minute correctly copied. Sounds like your problem may be that you are pushing the speed. If you can copy a limited number of characters before you lose your place, then slow down the speed a bit until you can copy a significantly larger number of characters successfully. In reading about the challenges learners have with Morse Code it seems to me that there is a belief that going faster leads to faster learning. Not so. Learners need to proceed at a pace which they can manage with comfort. Many learners seem to believe that characters should be sent at 20 wpm to start with, along with spacing that reduces the overall speed to 10 wpm or whatever. This simply will not work if you cannot recognize code characters at 20 wpm. Thus the learner needs to reduce the character speed to that at which characters are easily recognizable all or most of the time. If you proceed at a rate (in wpm) that works for you (correct copy for a minute at a time) you will gradually become more proficient automatically. A speed which you can copy correctly will in short order seem to be quite slow. At that point increase the speed by a couple of wpm. You will find that to be slow in a few days or a week. Then move along gradually until you are perfectly competent at 15 or 20 wpm at which time you should simply get on the air and your copy speed will automatically improve over time. While you are making contacts.