It may take you a few days to get used to it. Don't give up after one ride. Also position the mirror just to the left of your field of view. So that you have to turn your head left as if you were in a car. If it is too close to sraight ahead it can be annoying and bother your eyes if you use it a lot.
It took me a few days to get used to mine, lots of riders around here use them and tell me the same thing, give it time.
Now for about a full second or so, I can use one eye to see forward and one to see back at the same time. I've been using mine for years. I can also move heaad around to see at any angle behind me,