Nine times out of ten it's not your controller — it's your deadzone.
I've had three people this month ask me if their controller is broken because their car keeps slowly rolling to the left in Sideswipe. In all three cases the controller was fine.
Here's the deal — most Bluetooth controllers have a tiny bit of stick play that Android reads as movement. Sideswipe doesn't have a native deadzone slider because it doesn't officially support controllers, so you have to add one in Key Mapper.
Under the analog stick trigger, there's a threshold setting. Bump it up by 5% at a time until your car stops drifting when you're not touching the stick. For me that's 15%. On a beat-up secondhand controller it might be 25%. Anything over 30% and yeah, the controller is probably actually dying.
This one setting has saved me from buying a new controller twice. Try it before you spend money.