An opinion is a weak, mild form of belief. "It's my opinion that I'm a good person." What most people think of as a belief is somewhat stronger, "I believe I am a good person." A conviction is the strongest form of belief,"I'm convinced that I am a good person." I KNOW I am a good person is (in my opinion) a way of stating a conviction. A person who KNOWS he is a good person is someone who has more "references", real or imagined, as evidence than one who merely has a belief that he is a good person, let alone an opinion of such. Anthony Robbins wrote a marvelous chapter on Beliefs in his excellent 'Awaken the Giant Within'. I think you'd get alot out of it!

I like your affirmation 'I am a good person.' How about some other affirmations that are on the same theme and would support and build on the one you mentioned. 'I love myself.' 'I like myself.' 'I am a wonderful person.' 'I love life.' 'I deserve the best.' 'I have high self-worth and high self-respect.' 'I do things joyfully.' 'I feel good about myself.' 'I feel good about my life.' 'I have high self-esteem.' 'I radiate a powerful inner confidence.' Installing such truly empowering beliefs can only enrichen your life immensely!
Hope this was of help! --- Mike