What should I do if I want to learn seal cutting and I can't write at all, but my writing style is still very bad? ...

You don't have to be good at calligraphy to learn seal cutting. Buy a printed book, draw it on translucent paper with a tracing pen, then cover it with water and scrape it with your nails until there are no bubbles. After a few minutes, take off the paper and the words will cover the printed surface. Buy a book on seal cutting, it will be very detailed. What you have to remember is that seal cutting doesn't have to know calligraphy!