SA, I can see where you are coming from. It's not very nice being the 2nd choice. I would be hurt by that, too.
I also see that she didn't ask you if she could come along. She just assumed there would be a place for her. That's how it should be, but she's not the person to whom that reserved place has always belonged.
Maybe it will be good for her to see you are headed out there without her. The great thing is that you won't be alone. If she worries, so be it. Let her gears turn.
Was walking along the pier in San Fran this afternoon. It's been a beautiful day. Sailboats plying the waters back and forth across the bay. Do you sail out here?