"...bringing every thought into captivity"...II Corn 10:5. The more we pounder on our past failures the longer our stay in the valley of failure. The truth is failure is a part of every success story, it's meant to help us step up our game and do more better. Someone once said "there's nothing as a barrier when you are determined to breakthrough".
When you accommodate thoughts such as "i can...", "It's possible...", then you begin to see some pretty positive turn that your life would take.
Other people's "You can't" doesn't have the potential to stop or limit you but the moment you admit that you really can't then you're doomed and your bailout is you paying attention to those salient signs telling you "you can", might be your bible, probably the still small voice from within you, the fellow next door, that folk down the street or myself telling you now that you've wandered long enough in the "valley of you can't" and now is the time to take that one step that would make the difference.
I wouldn't know what you've thought or people termed impossible about you, c'mon with God in the picture and you playing your part "it's possible" and as regards that mountainous project "you can climb it/you can level it. Just stay positive and invest more effort, remember "you can do all things through Christ who strengthens you.
Believe it - then start thinking it
Think it - then start saying it
Say it - then you are on your way to becoming it.
Let me end with this quote from Oprah Winfrey "You don't become what you want, you become what you believe".
