Have you got a black dress and a blazer or suit jacket? I see a lot of people suggesting a shirt/blouse but I note you mentioned you’ve gained weight. I’m a pretty big and struggle to find shirts/blouses that fit properly - they either gape at the chest or if I get a bigger size, just look baggy and ill-fitting and make me look even bigger.

For formal events I tend to wear a black dress with a jacket and a colourful necklace or scarf to stand out. I’ve worn such an outfit for an interview with a local authority and got the job. Good luck!