Trying to work his way back

I have read many situations where they Hoover. Mine has on small scales over the past several months since leaving me for O W. I have been confused and heartbroken throughout this time as was married for nearly 20 years with 3 children. After reading, counseling and spending many hours on this forum I feel I have wised up to what is going on in his mind.
He has been saying for months he still has feelings for me and can't move on and after many months has started breaking away from OW. I have felt very strong of late and accepted his decision to be with OW and have finally felt some definite progress within me.
Please remind me of what he might be up to and what will happen if I allow myself to go back. How can I stay strong and be true to me. I have said i am not going down this road again but he does not seem to hear that. Please just remind me. And why would he want to come back if she was his everything?
Thank you