"Nationalism" is a code word used for socialism.
The difference likely lies in connotation. Nationalism is almost exclusively political in nature, whereas patriotism is often not. In effect, nationalism is the politicization of patriotism. It does not necessarily mean a belief in national supremacy, just as racism and racial supremacy are subtly different.
Nationalism in this day and age is simply the rejection of globalism, and on that, I firmly put myself in the nationalist camp.
If one word just becomes a halo word for all good feelings and the other a word for all the evil ones, then the distinction becomes more propagandistic or polemical than real.
Sure, as we usually understand the words "patriotism" is a better thing than "nationalism," but it's also more of a luxury of long-established states secure in the world. Nationalities that are still struggling to become independent and countries that are less secure and less powerful are always apt to be more "nationalistic."