Please do not write in and tell me not to install the wheels - I agonized over this so long, and am only doing it as a last resort.
I know you don’t want to hear this, but at least for children there has been a trend in recent years to skip training wheels completly. It just leads to children trusting in them and never learning to balance. Where did you find strong enough training wheels for a grown adult anyways?
Just like Daniel R Hicks in the comments I suspect the bicycle. I’ve never ridden one of those cruiser bicycles myself but the seating position generally looks horrible.
Can’t you borrow a more normal bicycle, adjust the saddle height (hiiiigh!) and ask someone to assist you i.e. keep you steady so you can reach speed? (it’s much easier to balance a moving bicycle)