Ok, Yume, :) Sometimes even korean use these words mixed. 결정 means you make yourself clear for doing action for something or attitude toward something. Or contents madeup like that. so we can use this word like '~에 대해 or 대한 결정을 내리다, /~ 대해or 대한 결정을 보다/~대한 결정을 짓다,/~에 대한 or ~으로 결정이 나다/~한 결정에 따르다. 결심 means 'makeup your mind for doing something firmly or that kinda mind itself' so we can see the similarity of these words but still there is a slight difference in the shade of meaning between the two words. hope it can help you out. :) if you have more question, please leave here. thanks. :)