One of the things that you should know about proclivity and propensity is that these two things just refer to tendency. They might seem very similar, but they do have a few differences that will help you distinguish one from the other. When you say proclivity, this will refer to something that is completely negative. If you would use propensity, this can also be used to refer to something negative.
Yet there are also times when you would use it for something that is positive. Proclivity means that you are behaving in a way that is considered to be immoral. When you say propensity, this means that you have a habit that you are finding hard to break. There are times when propensity can refer to a good or a bad habit, while proclivity will just refer to doing an immoral act.