What are you using for a bender? If it's not an all metal name brand traditional bender like Imperial, etc, you are wasting your time.
Don't beat yourself up. I would not be adverse to ordering a repop 3/8 tube, and there is stainless braided teflon options as well. I WOULD NOT RUN rubber unless it has a stainless inner braid
These types here.......come in various forms, more or less expensive, more or less "strong" some are single size and some stepped like this one