Patriotism is loyalty to a country, which is simply an area of land over which a government presides.

Nationalism is loyalty to a nation, which is a group of people with a shared history and culture.

Patriotism can be good, because all forms of civic engagement meant to improve one's country can be seen as patriotic; so both anti-Vietnam protesters and pro-military boneheads, who both believed themselves to be bettering the country, could be called patriots.

Nationalism really can't be good unless you buy into heterogeneity of a peoples as being good. Moreover, it is not simply an implication of "my group good", which can be true of multiple groups, it's a directly adversarial stance that implies "my group better".

The US isn't and has never been a Nation-state, i.e. a country made up of one nationality, so any profession of Nationalism towards America intrinsically implies the belief that only one group of peoples within it is the 'rightful' nationality, (which is why the current Christian Nationalism movement is inherently racist and White Supremacist, as well as intrinsically historical-revisionist).

Here are some thoughts by a political scientist on the distinctions, if you're interested.

A lot of you're an edgy teenager on the internet.

Nothing if you're a well adjusted adult.

Nothing really, many people just confuse nationalism with patriotism and claim to be patriots, while they are dumb nationalists.