This is strange, but for some reason this does not work for me.
I easily distinguish between V / 4 H / 5 and B / 6 when they are separate from other characters.
But as soon as I start doing exercises with a full set of characters, they are completely mixed.
I have been practicing these symbols for 12 hours, but there is still no breakthrough.
As Chris said, it is a general state of affairs and all of us face difficulties with those characters.
In my own case, I had a quite some difficulties with them and the turning point occurred when, instead of practicing sending and receiving with a paddle, I started drilling the characters with a straight key using a marvelous old RAF manual, A.W. Eley's "The New Morse Code Manual". He presents the characters by cognate groups:
Lesson 1 : T, M, O (-, --, ---)
Lesson 2: E, I, S, H (.,..,...,....)
Lesson 3: revision
Lesson 4: A, U, V (.-, ..-, ...-)
Lesson 5: N, D, B (-., -.., -...)
It built a very strong muscle memory for those characters with a straight key and I have far fewer problems recognising them when I hear them now.
You can find pdf copies of Eley's manual on the web, try using it or drawing from it for your practice and you may find that the sequence he suggest for learning the characters works better for you than LCWO's sequence. It really did for me.
You will hear it said that it is not a a good idea to start sending too early, before you have learned the code, as it will spoil your fist, but actually this was standard RAF training in the 1950s and seemed to work just fine.
I have personally found practicing sending and receiving with a straight key (i.e. you listen to a character or later on a group of characters and send it back) far more helpful than doing the same with a paddle.
I can see why the old-timers suggest starting with a straight key and, in my own case, I diligently use both.