"Nationalist Socialists" are very bad people, and that mixture gives the component "nationalism" a bad name. I'm the opposite of socialist. I oppose most socialism, even in its milder forms (which I consider a bridge to despotism). I think socialism can have short term benefits (which is why it is enticing to good people), but in the long term, it inevitably destroys economies and kills people. The only good socialism is that socialism found in monasteries (because monks don't act in solely self-interested ways like your average citizen).
If by "nationalist" you mean "statist" (e.g., support for greater power for the nation state, possibly even dictatorship), obviously, I'm not a nationalist. Look at the signature of my posts. I'm fundamentally a libertarian, which is the opposite of that. Also, I'm a federalist, and I would like to see power returned from Washington D.C. to the States...so I am not for a strong national government regarding domestic matters. In my view, the government should be weak domestically, but latently strong internationally...which brings me to the next interpretation of nationalist...
If by "nationalist" you mean a person who believes that each nation should look after its own people before it looks after the peoples of other nations (e.g., America First), well, that is an idea I can get behind, as it's the same thing as being a "family-ist": I look after my wife and kids first, before I think about charity for others. That is appropriate, and I would even say morally mandated. While acknowledging the legitimate call to charity for all, we should first care for those in our custody before looking to help others.
I never used the word "nationalist" to apply to myself, since it got a bad name. I never thought about it. Like you (I suspect), I was pretty shocked when Trump used the word. But, then, it kind of made sense. And, I suppose the POTUS should always be a nationalist (a patriot), even if he is a Democrat (at least a Democrat that is not a socialist). But, I would be quite worried if Bernie Sanders started calling himself a nationalist, because he is already a socialist, calling for the aggregation of power to the Federal Government. Trump leads a party that generally talks about downsizing government. (Granted, they don't seem to be doing that nowadays. I think both parties are sliding towards statism, which is why I think the US is in trouble.)