However, if you tell your attorney "I'm guilty, but I'm pleading not-guilty", that is still protected? Though I would assume your attorney cannot then put you on the stand knowing that you are going to perjure yourself. Correct?

So it was because the attorney was solicited to aid in the commission of a crime that it isn't protected?

Yes.

The main purpose of the attorney-client privilege is to protect client's ability to get effective legal services from counsel. That's an important public good, in the eyes of the law. It's an important public good even when the legal services are criminal defense. Sure, letting criminal defendants get access to good legal services does increase the risk that some guilty folks will go free. But it increases the fairness, and ultimately the legitimacy, of criminal verdicts if the trial approaches a fair contest.

That's not true, however, if the legal services the client is receiving is "how to commit crimes." There's no benefit - at all - to clients receiving effective assistance from counsel in criming or how to crime better. We don't want defendants to lack access to legal advice in a criminal trial for crimes that have already happened, but we do want them to lack access to legal advice in how to do future crimes.