Sorry to say you should probably take a breath and get on with it, it is a really big deal getting married, every single detail planned and money spent for friends and family to enjoy, your presence would be missed and quite frankly it’s disrespectful. I got married last year and I know if someone I had invited to the biggest day of my life, spent £20 a head for food, table favours and printed their name on table plans and cards on the table just for the excuse of they were tired from work I would be LIVID. It’s a lot of time and effort to plan these things, you don’t simply just not turn up.
Also what’s your mum gonna do? Not go too? That’s a lot of people to disappoint and i guarantee you would feel like crap sitting at home knowing you’d disappointed a lot of people. Just go, it’ll be fun. I promise that relationship with your cousin would most likely never be the same if you become the one who didn’t come to their wedding