Past crimes, when there’s no reason to believe you would commit another crime in the future are covered by confidentiality. However if the lawyer believes you intend to continue committing crimes or that you have admitted that you plan on committing a crime are not covered. So yes, your lawyer could be a witness against you if you admit to planning a crime you have either not yet committed, or which you hadn’t committed at the time you told your lawyer but subsequently then committed.

There’s also a question on whether admitting to crimes unrelated to your current case is covered by confidentiality or not. I’m not entirely clear on if that applies, but I think E.G. if a lawyer is representing you on a robbery case and you admit to him you murdered somebody 5 years ago he might be allowed to tell the police about that without breaking his obligation of confidentiality since that admission is entirely unrelated to the current case.