do you have anything that you have found to help ground from the thoughts/urges? I know for me, art helps a lot, or journaling on my phone if I cannot do the art. music helps ground in the moment sometimes, as does sometimes indulging the fantasy in my head (imagining I am following through on the urges and trying to get the feeling without doing the act. sometimes this makes things better, sometimes more unbearable, but thought I would throw it out as an option for you just in case it is something that may help you). I sometimes reach out to a crisis chat, but that needs to be done via the computer because very few are compatible with phones. I have a very difficult time speaking to someone about any of this stuff, so I prefer to write it, but you can also call a local crisis line and they can be in-the-moment help with getting over the urges. Can you try to get lost in the wedding festivities? One thing I have been told to do is tell myself "I am here to enjoy (insert important event here). I know I really want to self-harm right now, but I also really want to enjoy this. I am going to be present at (whatever event) and think about self-harm later when I have some time" I find that giving myself permission to deal with the self-harm at a later time helps ease the thoughts in the moment and I am better able to be present. This only works for me for "special occasions" but it is worth a shot. I knwo the more I tell myself I cannot do something, the more I end up wanting it. If I give myself permission to do it later or think about it later, it lessens a bit. This is actually what got me through a vacation recently that preceded a huge change.
Do you have anything planned for your coming weeks off? More therapy? a treatment program? anything to help with your current issues rather than just languishing? Would it help to make a plan for the time off if you don;t already have one?
Is there anyone at the wedding you can "hook up with" in terms of socializing and keeping busy? Even if you don't tel them what's going on, having someone to hang out with can be helpful sometimes. Also, if you have a therapist, can you place a call and maybe get some interim support to help you through the weekend?
I'm sorry you are struggling with all this. It can be very distracting, even when you are trying to enjoy something truely fun. Hang in there and keep reaching out.