My two cents.

After entering the door Roland sees himself as the teenager he was on that day with Marten and his mother. Knowing what to do he wishes to go back to that day. I always felt (it is also stated in the book I think)that day was the key moment.

Imagine Roland had never encountered Marten and his mother.
It may sound weird, but I think Roland would be way more happier with a life inside the walls of Gilead and fighting not knowing they would lose the war even though they would won the battles. Not knowing about his unfaithfull mother and his doomed world.
Roland would be happy, I have no doubt about it. He would live a more human and simple life. This would be the happy ending for Roland in my mind.

For myself I can't imagine a worse ending then this.
But yeah it would work for Roland.