Someone who is firm and determined in a belief or a position can be called steadfast in that view, like your mom when she thinks you really shouldn’t wear that outfit.
The word steadfast traces back to the Old English word stedefæst, a combination of stede, meaning "place," and fæst, meaning "firmly fixed." Picture a steadfast person standing firmly in place, not wavering or budging an inch, and you’ll have a good sense of what this word means. Someone can be steadfast in a belief, an effort, a plan, or even a refusal. Whatever it is, it means that the person will calmly hold firm to the chosen position and follow through with determination.