Hi Cetti - I know what you mean, it's hard to keep a lush green plant without it developing rust. You need a very large tub for each plant - I've found partial shade is best. Some varieties do better than others, black peppermint is good but a little strong & is used in teas a lot, pineapple mint is a lovely variegated one but an acquired taste, and Bowles (the hairy one) but it often gets mildew instead of rust....you can't win!
Keep picking it to encourage new growth, feed once a month, pinch off any leaves showing rust spots & the flowers to try & keep them as strong as possible.
Oh, & take cuttings, they root in water, so as to keep a succession of plants.
Or buy a jar of mint sauce! :o)