How do I politely decline an unwanted invite?

I was invited to my boyfriend's friend's fiance's bridal shower. Just me. Not him. I suffer from social anxiety (am trying my best to get treated) and very much do not want to go to this bridal shower ESPECIALLY because it was only me who was invited (which is expected - bridal showers are usually just women). When I first got the invite I had a pretty bad panic attack over being invited. I have no idea how to decline this invite. I kept trying to think and make up random excuses not to go but I have nothing. I literally have no excuse not to go. I just am very nervous & anxious about the whole situation. My question is....how do I politely decine this invitation? I have to RSVP soon and am at a loss as to what to say and how to say no. I'm obviously still going to send a gift & card. I just know it would be an uncomfortable experience for me because I am not very close with the bride and don't really know anyone there except her. The bride's maid of honor is the one throwing the shower and the maid of honor is the one I have to RSVP too via email. Is there anyone who has better social skills than I that can help with this?

Hi, It’s good to push yourself out of your comfort but if you’re panicking about this then clearly this isn’t for you and that’s ok don’t be too hard on your self. I can’t understand if you don’t know anyone. Since it’s by email and not directly to her, it’s a little easier you say you can’t go but you are sending a present and card, its a nice thing to do :-). When you try to make up excuses they often don’t sound true anyway. It’s good to push your self out of your comfort zone from time to time, I’ve not been to a bridal shower, but it’s probably not the best place to start. I’d say start small and take your time.