If I remember correctly, you are the young lady who also had some trouble with legs out front, right? and that was due to the saddle size/fit. I would not worry too much about it. It's a small habit and you'll rebuild body memory. If I were your instructor I would certainly not yell at you regarding diagonals. They only really matter if you are going in a circle. and even then, it doesn't really hurt the hrose. it just makse it easier for them if you are on the correct diagonal on a curve. on a straightaway, it makes not a whit of difference.
There is so much that is more important than having the correct posting diagonal, IMO.