Rank gains/losses are going to be based on your performance compared to other players using that same hero within a similar rank range. Playing on Mercy at that rank is going to dedicate you to basically have to be a healbot unless you’re a REALLLLLLY good Mercy player that got placed way wayyyy too low (think a GM Mercy who got bugged into Bronze that can go around and pistol a bunch of Bronze players because they can’t aim).
Also, because the ranked system was bugged and placing a lot of players in way too low ELOs by mistake, the data pool is currently skewed. You have players that were even in the Top 500 who got placed in Bronze 5 because of a bug so you’re having your performance erroneously compared to these players who should not have been there in the first place.
As of right now, just do your best and hope Comp data gets balanced out in the next week or two. If you can, maybe try playing a hero like Moira until you get out of Bronze since she can get a lot of value very easily at that rank. I’m not sure but your performance could look better on her which will make the game rank you up faster.
Sorry for the long reply but hope it helps and good luck!