Having trouble downgrading to High Sierra from Mojave

I’m trying to downgrade to High Sierra from Mojave on my 5,1 mac pro. I can’t get anything to load. I tried to install from the Boot USB but it goes very slowly and just hangs at the full loading bar. I let it run over night just in case it was still doing something but it was in the same place in the morning. I’m desperate at this point.