Canadian with Green Card crossing the border back to Canada after a felony? Can I get back into the US?

4 years ago, I was arrested with a small amount of felony narcotic on me. (Under a gram.) I am a Canadian, living in the US with a green card and have been living here for 19 years with no prior arrests...not so much as a speeding ticket. The case went to court and I was ordered to do 3 months of NA meetings, then report back. The case was then closed and I was informed that it should not affect my Green card status or crossing the border. My lawyer told me that it would simply remain within the court system and not affect other agencies. Is this true? My brother is getting married in 2 months in Canada. Can I safely cross to Canada and safely cross back? And when my Green Card comes up for renewal will I be able to renew it?