To maintain a good mental attitude in English, you can say "keep a positive mindset" or "maintain a good outlook." These phrases come from the idea of having a mental attitude or perspective that is focused on the good or optimistic aspects of a situation, rather than the negative or difficult ones.

For example, you could say "I'm trying to keep a positive mindset about my upcoming exam" or "She always maintains a good outlook, no matter what challenges she faces."

Here are some more examples of sentences with similar meanings in English, along with their Chinese translations:

"I'm trying to stay positive, even though things are tough right now." (我正在努力保持积极的心态，即使现在的情况很艰难。)

"I think it's important to maintain a positive attitude in order to be successful." (我认为保持积极的心态对于成功来说很重要。)

"She's known for her ability to stay optimistic, even in the most difficult circumstances." (她以能够在最困难的情况下保持乐观的能力而闻名。)