Take this FWIW but the lightbulbs in my head went off like a Christmas tree when I read about that exchange you had in the car with your W.
You two might as well have been me and my husband when he moved back in but I was still wrapped up in OM. I was trying to shake the feelings I still had for him at the time, trying not call him...etc....and YOU and YOUR W had a conversation in the car that MIRRORED damn near word for word what used to go on between me and my husband. In fact, to be completely honest, YOUR words were the ONLY words that were different. Truth is GH, MOST of the things she is saying to you now are VERBATIM the same BS I used to say. VER-FREAKIN-BATIM, I SWEAR.
Now HERE is what will change that...
Stop asking questions. Do not call her cell phone when she is out alone. In fact, if it is not about the kids or something else VERY IMPORTANT DO NOT CALL HER CELL PHONE AT ALL. Don't suggest a date. Don't ask her what she's thinking, feeling etc... Don't ask about the future. DAMN SURE don't buy a ring for her based on the fact she SAYS that means things will be okay.
EXTRICATE yourself from her emotionally EXCEPT on the most basic level.
You know I do not have a habit of posting to you often. I'm telling you your wife acts more like I used to act with every post I read.
I know you're going to keep doing your own thing but I am asking you to carefully consider the things I am suggesting here.
You're never going to make any headway until she no longer thinks you're a sure thing.