I set my mind to learn English well.
我下定决心要把英语学好。
I set my mind on believing you, but you cheat me again.
我下定决心相信你，但你还是骗了我。
I set my mind to forget my past，but you always remind me of that sad memory.
我下定决心要忘记过去，但是你经常向我提起那些伤心的往事。