To both you and fella: "heresy" is not an insult. It is a descriptive category. It means "error" --- without any implication of personal belittlement or contempt. It does not mean "You're dumb, you're depraved" or any such thing.
Modalism is squarely within the list of historic heresies, as would be Arianism, Unitarianism, and-many-another-ism. Many great saints made perfectly understandable human errors in the theology department, and if they were unaware of it and not contumacious about it, it was not even a sin on their part.
I used to be attached to a whole catalogue of heresies. If I search around, I still might be able to find a few little ones around here somewhere... :o)
But I'm sorry if it came off as a hard word. You have my goodwill, believe me.
I will pray for your "T" key.