Ephesians 4:23 says, “And be constantly renewed in the spirit of your mind [having a fresh mental and spiritual attitude” (Amplified Bible). What got my attention from this translation is the word “constantly.” The mind renewal happens over and over again, not just once.
Likewise, Romans 12:2 says to “be transformed,” but, as I understand it, the tense of the word “be” reflects an ongoing process. Your mental transformation doesn’t happen in one fell swoop. It happens in steps or stages, through your life.
Isn't this freeing? Through life, God actually expects you to make mistakes. He's all-knowing, so He knows you're going to make mistakes even before you know. And He loves you, anyway.
But it's what you do after you make mistakes that determines how quickly you're transformed. God doesn't want you to meditate on your mistakes or beat yourself up with guilt and shame. Doing that just causes your mind to get "stuck" in a negative place and distracts you from moving forward. Think about it. How are you supposed to be "continually transformed," when your negative, self-condemning thoughts act like an anchor, holding you back?
The Bible tells you what kind of attitude you should have after you make mistakes. It simply says you should confess your sins and repent from them. God forgives "seventy times seven" times, so there's no logical reason for you to get stuck mentally revisiting negative thoughts.
You can't be transformed [continually] when you're focused on your mistakes, judging yourself over them. You can only be transformed by God's healing truth, so meditate on Scripture instead of your negative thoughts.
In regard to weight control, if you mess up (e.g. eat too much or eat when you aren’t hungry), just pick yourself up, dust yourself off, get back to reading Scripture and filling your mind with Truth, and get back to making the right decisions, continually.