So the point here is that when we cut ourselves down, we are rejecting the Word of God. We are also second guessing His wisdom in His making us. Sure, we mess up what He intended for good. We are not always good stewards of our bodies that He blessed us with. But the original creation, as designed by Him, is very good. So for us to think of ourselves "unworthy" flies in the face of how God views us! He knew us before we were born and cares about every hair on our heads! And if we've accepted His son as our Savior, God sees us in our PERFECTED state! So for us to run ourselves down is ungrateful and dishonors Him.
I do think we have to balance this with understanding the gifts God has given us though! No amount of surgery or prayer can turn a 5'4" 48 year old non-athletic woman into an NBA star. However, He can use her to encourage the future NBA stars that come across her path!
People often say to me, "you ran a marathon? I could never do that!" Really? Can't? Or not willing to try? Check out this video before you give that answer.
I can do all things through Christ Jesus who strengthens me. (Philippians 4:13)