score:4
At this point, with your extended absences away from the US, and your intent to live outside the US, your green card is effectively invalid. You would now have an even harder time convincing a CBP officer that you will return to live in the US when you clearly aren't.
Given that, I would suggest voluntarily surrendering your green card the next time you enter the US. Don't wait for them to ask about it, because then they'll think you are trying to hide something. When you surrender it, they may actually take it away or they might clip a corner and give it back (either way the result is the same). Then they will (hopefully) process and admit you as a Canadian citizen without any further drama.
Consider that even if you conceal your green card this time (and that they'll probably already know you have one anyway), you'll have the exact same problem next time you enter the US.
Upvote:4
Assuming you only want to stay in the US for a short time, simply showing your Canadian passport will be enough to legally get you entry. You are not obliged to mention your Green Card status unless asked. However make sure you answer any questions truthfully, especially if asked about any visas or such like. They probably know about it anyway. This approach will get you into the US for your short term visit, which is the question you asked.
Whether this approach will affect your Green Card status, or what effect your long absence will have, is off topic on this forum, and if you are concerned you should ask about it on our Expatriates site.