Handwashing Sweat Stains and Underwear

Hi, I don’t have reliable access to a washing machine (like once a month and can only do a small load) and dryer anymore and I can’t go to a laundry mat. So far I’ve been good about washing clothes that have no stains and have only been worn lightly, but I’m having trouble washing my partners shirts that have crusty armpits as well as I would love advice on how to go about washing my underwear with discharge crust. I’m not sure if I just haven’t been agitating them enough or not. I’ve seen the bucket and plunger method but don’t have access to a bucket or drill anytime soon so all I have is the tub or sink and a potato masher and my feet/hands haha. Thanks so much :)