It's been EIGHT years since he's been separated OP!
If he really cared for you he would treat you much better than this. He'd be emotionally accessible, more personally supportive, actively planning a life with the two of you together, looking forward to the years together, doing financial planning together, etc.
He would be looking at freeing himself from a redundant relationship (ie his marriage) IF he really wanted a loving, open and honest life with you.
I’ve asked him on numerous occasions as to why and all he says is I will do it tomorrow.
This ^^ wouldn't be happening.
He is, it seems, happy with the way things are - the status quo works for him.
However it doesn't work for you. It's been 3.5 years since you met him and nothing is changing.
Why are you wasting your life and hanging on?
Surely a life without him in it would be emotionally more peaceful than this "No Man's Land" that you are in?
Courage OP and cut him adrift.