I vote for the thinking too much.

Look, we aren't in pain now because we did something wrong ten years ago. It just doesn't work that way. You are in pain simply because you have feelings for your therapist and her upcoming nuptials feel like a loss to you. That's okay. It probably isn't all that unusual a reaction to that kind of therapist news. I doubt that you are particularly unique in this reaction/experience.

Her marriage isn't about you at all. She didn't do this, honestly, with you in mind at all. You are applying a false connection between events here. Nothing has honestly changed for you. You weren't in a personal/romantic relationship with your T to start with, so this news really isn't changing that status.

You do need to be honest about how you found out the information (you went a bit far with your FB stalking there), and clearly you need to work on these transferential feelings about her. But try not to attach addition meaning, like the world is punishing you, etc., that just isn't there. Come down to earth a bit.