It is not obligatory that a person or a country that is nationalist becomes fascist over time, so the answer is yes, it is possible.
Nationalism is a healthy love for one's own nation, for its culture, beliefs, language, traditions, and is proud of them, but not on the expense of the other nations and to consider them as people of lower order, or primitive, not worthy to live. The fascism on the other hand, is an extreme form of the nationalism when we have the glorification of the nation and its values, but in a oppressive and aggressive manner towards the other nations. There are multiple examples for nations where the general population is nationalistic, but has nothing to do with fascism, and in fact is appreciative of the other nations and have good relations with the other nations, some of them are Finland, Estonia, Poland, Hungary, Denmark.
