Switch to natural deodorants and stop using antiperspirants if you can. I used to have constant sweaty pits and pit stains all the time, then tried just deodorant and after about a week or so adjustment period it’s been remarkably better. Way less pit stains and it’s much less severe of an issue too. My tshirts have been faring much better as a result as I used to have to toss them after a year bc the permanent staining would be so bad