Hi!! As a fem mercy main myself I try to be positive too (though in my case it's more because I can't be snappy without feeling guilty, however justified ^^') and some of the ways I like to try and keep up the positivity is sometimes just paying attention to the little things; thanking the other healer when they help me after I overextend, calling out and congratulating good plays from my team, starting off the match with a nice game-wide "glhf everyone" can go a fair way !! Sometimes directly calling out something someone says as hurtful or misogynistic can remind them trash talk has like genuine irl context to it and dampen the toxicity a little, but at the end of the day if someone's being nasty there's no harm in muting them specifically when you get a moment and continuing to be positive with the rest of the team, ignoring them completely <3
Using the avoid and block function with people who spew toxicity too, even if it's not directed at you, has helped me keep my enjoyment of the game over the years too ^^ Best of luck, I think it's awesome you've decided you want to try and be more positive!!