Yes, you can get out of it. No one should ever feel obligated to attend the wedding of a business colleague, coworker or manager if they do not want to, do not feel comfortable going, or for any other reason. If you don’t want to go, RSVP No and be done with it. You have other plans (like binge-watching your favorite show on Netflix). But, let’s get to your particular predicament so I can give you some advice.
Quite frankly, I'm surprised your boss sent a wedding invitation to you at all. However, to answer your question, you do not need to feel obligated to attend. Why were you invited? Who knows. Sometimes brides do silly things like invite people so they can fill up a room, or to flaunt, or make amends. That doesn't seem likely because that ship has sailed. Or perhaps your boss felt obligated because she sent you a Save the Date and it would be rude to not follow up with the invitation.
Ordinarily, a simple checkmark for “No” would be the easy way out. But, the tricky part is that RSVP marked “Yes” that you’re carrying around. It would be tacky to scratch through the “Yes” and add regrets. Here is my advice to bow out gracefully, and not damage your relationship any further:
Get a blank note card, and write a short, concise message stating that you are unable to attend and wish her all the best. Perhaps something like this sent to the RSVP address:
Thank you for the invitation, but I am unable to attend your wedding on the June 8. Please accept this note in lieu of the RSVP card you sent. I'm sure your wedding will be beautiful, and I wish you all the best.
Then sign it, stamp it and mail it. There is no reason to mention the RSVP card, or lie and say you lost it. Sending a separate note gives a connotation that the card was misplaced or burned (a little Yom humor) so no explanation is needed.