Another possible cause of thinking you're singing wrong or out of tune:
    The sound outside the head, what others hear from my mouth is not EXACTLY the same pitch as what I hear in my head. I know this because what's in my head is usually louder to me, so it's easier to tune that. But then I end up singing flat. When I consciously pay attention to the sound coming in through my ears instead of the "inside sound" my tuning is great.

If hearing is damaged, putting hand firmly to ear as if to say "speak up" will help get more of the "outside voice" into your ears. Both hands is even better. Then it's practice, getting co-ordinated.