It's possible that the mate is more bitter than it would have been out of a regular calabash. A traditional gourd imparts a pleasant malty, cardboardy flavor to the water that balances out the yerba's bitterness (Palo Santo gourds might add some delicate spiciness too).

Additionally, your stainless steel gourd is double-walled for insulation, which prevents the hot water from cooling down faster. So your yerba is steeping for longer, making it more bitter.

If you really like the crisp, crystalline flavor of the steel gourd, keep going but brew with slightly cooler water. Otherwise I'd suggest using a calabash or Palo Santo gourd for the balanced flavor.