Green Card No Expiry Date; Lived in US Then Moved to CA

I need to know what my rights are. Here's my situation:

I have a Green Card with no expiry date that was issued when I was 4 years old in May 1979. I stayed in the US for several years then moved to Canada. I currently have US credit cards, US bank accounts, and a US mailing address. The businesses I ran in the last few years were online (from Canada) and generally sold from the US warehouses to US customers. I have never filed a US tax return. I do not have a SSN but I have an ITIN.

Right now I have a Canadian passport.

What are my rights with this Green Card? Did I automatically gain citizenship after staying in the US for a few years (even though I never applied for it)? Technically it probably expired when I left the US. I would be interested if it could get me a SSN or if I could still use the card in some way. There is no question that an SSN could come in handy when applying for US credit...

You are NOT a US citizen. unless possible if your parents were US citizens. But even then you have to apply for that.
To maintain your GC, regardless of its expiry date or lack thereof, you have to live in US, or have permission to live outside for up to 2 years.

How have you been entering US? Legally you MUST present your GC when entering US or it is considerred abandonment, and entering US with GC while living outside, would normally lead to questions from the border officer, and ultimaltely confiscation of the GC, which they would deem abandonned given how long you've been outside.

Taxwise, you should have been filing a tax return in US every year (or that too is considered abandonment of GC) and are liable for 6 YEARS back taxes and penalties -- even if your GC is dead at this point, ironically.