First, you need to stop worrying about being out of fashion -- or thinking about fashion at all, really. Use your own judgement about what suits you personally and what has visual appeal.

I do think Muirenn's advice about proportion makes sense.

I have used tiny bags with a shoulder strap for dressy events, but you have to make sure the strap looks okay with your outfit and will stay on your shoulder.

If you have access to high-end thrift or consignment stores, purse/tote bargains abound.