I've followed your situation since the beginning and feel compelled to contribute.
I have to concur with dlt1's last comment. The next time she extends an offer, politely decline because you have other plans (or if you're with your son, that you don't feel like it or aren't in the mood). Don't engage in any relationship talk and don't volunteer any details regarding your plans. Don't give her the satisfaction of knowing that her mind games, which they might very well be, are affecting you. In order to be truly detached, you need to keep your feelings to yourself at this point. I know quite well from experience that an aura of mystery regarding one's life can really make the LBS question what he/she is missing. Go back to your picnic - you don't need to tell her where your picnic is or what's in the picnic basket.
