Short answer: a confusing something and a confused something are similar, and closely related, but not necessarily the same.
Let's try a simpler verb: interesting--interested. There are 3 people in this example, A, B, and C. Suppose that,
A interests B. -- Let's say that A successfully gets B's attention by making himself or herself look interesting. We can say that, B finds A interesting.
C may observe that,
- A is an interesting person (to B), and
- B is the interested person.
C may or may not think that B is interesting. However, if C thinks that the fact that B is interested in A makes B interesting, C may think that B is interesting, too, which in turn makes C think that,
- B is the interested person, and
- B is an interesting person.
In this case (to C), B is both interesting and interested.
The same applies to confusing and confused in your question.
When we say or write confusing ideas, we mean that to us, the ideas are confusing, and we look at it from our point of view. We feel that they are "confusing". To us, it's difficult to understand.
When we say or write confused ideas, we mean that the ideas are confused. They are not well organized or explained. We may say that the person who states the ideas "confuse" them, and as a result, the ideas are not clear or not easy to understand.
Thus, a confusing something may not be confused.
And a confused something may not be confusing, either.
In short, they're similar, and closely related, but not necessarily the same.
I think as most of the sayings go back to the olden days, old English, other languages, or different dilects, some of them seem to be incorrect grammatically.
The subject saying isn't common in use, it was used especially by some famous politicians, John F. Kennedy being one of them.
I think the OP is right that we should use don't ask instead of "ask not" to make the saying modern in English, but if you change the words of the saying, it'll spoil its beauty.
Best Answer
The difference is subtle but fairly clear. To say, "I am confused", is to draw attention to your state of confusion. It would ordinarily be used if you want someone to clear up whatever issue confuses you. "I feel confused", on the other hand, is drawing attention to your feelings or emotional state, and you'd use it if you want sympathy more than clarification.