A short, sweet lesson: Being a morally good person involves doing your moral duty. If you don’t at least accept that, then I think you’re basically mistaken about morality in theory (I say “in theory” because I’m comforted by the fact that you probably live as though I’m right).
But how does being morally good involve doing your moral duty? Here’s is where Kant has something important to tell you. Being a moral person is not just acting in accordance with your moral duty. You might live your whole life in accordance with your moral duty and yet in some really important sense still not be a morally good person. How so? Here;s how: It could be a very unlikely coincidence that your life is lived in accordance with your moral duty. Or maybe you are doing what you do because you’re getting paid to do it, and you don’t really reflect on what your moral duty is. Or maybe you, perversely, think that what you’re doing is really contrary to your moral duty and yet you desire to do it anyway. Or maybe you have some other motivation – you might not take advantage of a woman because you fear that it will harm your reputation with women, for example.
This is what Kant tells us, and he is right: Doing the morally right thing is where you act, not just in accordance with your moral duty, but you also act out of duty. Nobody is worthy of moral esteem for doing a thing that, as it turns out, is in fact morally right. You are only worthy of moral esteem for doing the right thing, whatever that might turn out to be.
And that is the gift that Kant has given you today!