You may not be getting physical attention from this woman but your mind is turning to her, and likely this is an element of fantasy - you already mentioned the idea of 'the one'. She is giving you attention (albeit casual talk and a smile) without any of the worries of a real life long term relationship - it's a kind of escapism possibly.

It sounds like you and your wife have a lot you need to discuss. I think it's a really positive move you are making things for her and planning a meal, personally I think these small loving actions are very important in a long term relationship.

Rebuilding the emotional intimacy (by talking openly and without reproach, and small acts of loving kindness) is a work in progress, it will be small steps and may not seem at first like anything is changing but steadily you can rebuild it if you both make each other a priority.

As for your thoughts about this woman, well you can try techniques like the rubber band or replacing your wandering thoughts about her with ones about your wife and daughter to be, those are good suggestions IMO. But try not to stress out about it, they are just thoughts you are having, you aren't acting on them, acknowledge them and let them pass. These thoughts do not mean you are a bad husband or your marriage is doomed.