Let's start by noting the similarities between the two ideologies. Nationalism and patriotism are similar in that they both promote a love for one's country and promote unity based on shared characteristics. However, they differ markedly on several other fronts. From a paper by Ali Altıkulaç and Alper Yontar (source: https://files.eric.ed.gov/fulltext/EJ1234371.pdf):
Patriotism has a defensive nature both militarily and culturally. On the other hand, nationalism may not be held separate from the wish of ruling. The unchanging purpose of every single nationalist is to provide more power and reputation for his nation rather than himself.
In other words, while patriotism tends to be more docile in its love for country, nationalism seeks to maximize the state's power over its own people and others. More worryingly, nationalists both historic and current have embraced ethnicity and race as the basis of the nation-state. Nationalists tend to believe that racial and ethnic homogeneity, not diversity and multi-culturalism, undergird successful countries. This is why nationalism has a bad rap because nations that have pursued nationalist forms of government have been associated with some of the worst atrocities in human history; the fascist governments of WWII are great examples of this phenomenon.
Lastly, I'll note one more critical difference between nationalism and patriotism. Patriotism emphasizes unity based on ideological grounds. In the US, that manifests in support for liberal ideas such as active citizenship and support for liberal institutions (e.g., three branches of government, independent press, robust higher education), which most Americans tend to agree with. On the other hand, nationalism emphasizes unity based on race and culture. Alex Nowrasteh from the Cato Institute explains how this manifests among American nationalists:
According to the Democracy Fund survey of Trump supporters, American Preservationists [nationalists] are 20 to 50 points more likely than other groups of Trump supporters to say that it is “very important” to have certain ethnic and cultural identifiers in order to be truly American. Of those ethno‐cultural characteristics that they value, 69 percent say being born here, 67 percent say having lived in America for most of one’s life, and 59 percent say being Christian. Most disturbing, 47 percent of them said that it is very or somewhat important to be of European descent to be truly American.
To answer your second question, I seriously doubt that nationalism can be a force for good. Its history is inextricably linked with violent militarism and bigotry against "the Other." There are reasons why a heavy stigma is attached to nationalism.
For further reading: https://www.cato.org/cato-journal/spring/summer-2020/case-nationalism-how-it-made-us-powerful-united-free-rich-lowry
