No, fascism and communism aren’t “opposites”

I don't believe politics is simple enough to allow opposites, but if there were such a thing, those two ideologies would be pretty close. Fascists are ideologically anti-communist and communists are always among the first they mass murder. Communists (along with anarchists) are consistently the foundation of anti-fascist action.

while communism is a highly ideological philosophy that’s never existed

"Yes, and,"

This is where terminology plays tricks:

• A communist society is the ultimate goal of the ideology called 'communism'.
• You're absolutely correct that no country has a communist society; in fact, it's a contradiction, since a communist society is stateless by definition.
• The countries that are labelled 'communist' (by themselves and others) are states with a communist government in power. This strategy of vanguardism is strongly debated among communists: many would agree with you that it's a contradiction, while others consider it a necessary transitional phase in order to defend from capitalist counter-attack. If we assume that the vanguard government is not corrupt (and we shouldn't assume that without evidence!), then it's a government that aims to create the material conditions that would cause itself to wither away, piece by piece. Obviously none has succeeded in that goal, but it's not wrong to call those governments 'communist', in the same way a person who supports socialism is called a 'socialist' - it's about a school of thought, about ideology, rather than describing the current situation they govern over. And to characterize authoritarian communists as fascist is ignorant about how fascist systems develop - fascism works to kill socialism and liberalism with the backing of the owning-class. No matter how many similar characteristics one may try and find on the surface, the two concepts are foundationally incompatible and opposed, and will act very differently. It's fine to hate them both, but they are not related.