Starting 3 yrs ago, I forgot how to enter my car. Is this dementia?

We're not doctors. A doctor would test you for all sorts of other medical, physical and neurological issues before concluding that you have dementia (and then why type).

Please please go to your doctor. What if it's a brain tumor? How does asking on this forum help you?

You have asked prior questions, one about falling backwards. It was recommended that you see a doctor because this could mean Parkinson's which goes hand and hand with cognitive decline.

Again, you need to see a doctor and have labs run and a good physical. You are coming to a forum of lay people from all over the US and other countries. We cannot diagnose ur problem. YOU NEED TO SEE A DOCTOR AND MAYBE A NEUROLOGIST.

So you cannot get into the car door, but you are well enough to come to AC, register yourself, and comment rationally. That's interesting on the fact of it.

You mention one thing. A car door and difficulty remembering how it works.
Can you tell us about other things you are noting?
Have you considered taking this up with your own doctor who is familiar with you?
With friends and family who know you? Have they noticed changes if you ask them honestly?

I think a Forum of strangers across the world may be the last to know what is happening with you. I would seek help close to home, and I wish you the very best. Hoping you will update us as you followup on this strange happening.