Why do I always sing in falsetto?
Singing in falsetto (or any register) is a conscious activity. You just need to be mindful of how you are singing and not do things you want to avoid. This may also mean consciously practicing other behaviors to erase previous muscle memory, but in the end, it's just some version of being more conscious and mindful.
Is it bad to sing in falsetto?
Generally I don't recommend using falsetto because of its' limitations. But it's ok to use as a stylistic choice if you choose to. It's not ok if you have to use falsetto. If you tend to flip into falsetto it's likely that your vocal type is Flip-Falsetto or Pulled Chest-High Larynx.