Hello
I think you just got a bit mixed up with the decrease numbers.
It can be fixed. I suggest either putting in a life line before ripping out the few rows to save the stitches (and you can leave the life line in until you are sure you have the shaping right), or to tink back the few rows. Personally I would tink back.
There are videos for life lines and tinking if you need them - let us know if you need one but perhaps you already know how to do these.
55 stitches on the needle
Mark off centre 13 stitches
21 - 13 - 21
(You may have over cast off as you had 19 and 20 remaining, you need 21 sts each side marked. When you actually work the row you do the arm edge dec of 3, then 13 centre cast off. Leaving 21 and 18 sts)
The shoulder edge decreases read as:
“On each edge cast off 3 stitches twice and 4 stitches once”
This is only the shoulder edge
In numbers for each side this is 3 + 3 + 4 = 10
10 decreased at shoulder edge
Each side has 21 - 10 = 11 sts
These are the 11 sts you are asked to cast off on the neck edge.
Because you have to begin the shoulder edge decreases just before the centre neck cast offs you will work
3 cast off at shoulder edge (this side now left until later)
Cast of centre 13
Work to end, now work this side only (21 sts)
Cast off 3 at shoulder
Cast off 11 at neck
Cast off 3 at shoulder
Cast off 4 at shoulder, no stitches remain
When you rejoin yarn to work the other side you will already have decreased 3 at shoulder so 18 sts
Cast off 11 at neck
Cast off 3 at shoulder
Cast off 4 at shoulder, no stitches remain
When you cast off a stitch is left remaining on the right needle which is still counted as a stitch. It means to cast off 3 you actually work 4 stitches off the left needle but 1 remains on the right needle after passing 3 over and off. Don’t forget this stitch.
I hope this makes sense. If you need more help do feel welcome to keep asking until you are successful with this tricky bit.