I often tell my students that if it is "important to" someone, it just means that that person values that quality or aspect. If it is "important for" someone or something, it can mean that whatever is important can progress someone's cause in some way. Sometimes both can be used depending on the context.
Being slim is important to me (I value being slim; I think I look better etc...)
Being slim is important for me/my career/my health (Being slim helps my career or improves my health)
Can you tell me what your example should be now?
Hope that helps!
- For Teachers