Hmm, I hope i don't confuse you more here. it's hard to explain but you are right, they are interchangeable. depending on the meaning of your sentence it can be used to describe your sentence differently eg: It's important to me to stick to my diet (shows urgency, determination and ownership, to who is it important? to you) It's important for me to stick to my diet (stating that it is important, not necessarily who it is important to, i could have told you it's important for you to stick to your diet, and then you would just repeat that, but not actually be owning the statement as true for yourself).