Get a piece of EMERY CLOTH (e.g., 120 grit) & "scrub" the braking surface of your brake pads AND clean your rims after-or-before each ride.
Regardless, your FRONT brake is more important.
NB. Check your brake AND derailleur cables ... they may need lubing/replacing.
FWIW. It sounds as though your BB needs maintenance (if loose bearings) OR replacement (if cartridge -- there IS something you can do, but it's more than most are willing to endure). In the meantime, lay the bike on its side & put a few drops of HOUSEHOLD OIL (e.g., 3-in-1) on the spindle where it exits the bottom bracket ... turn the crank 360º ... wait ... put the bike on the OTHER side do the same.