There are many things I cannot do. I'm simply not strong enough. Sometimes, I try and do them on my own anyway, and life reminds me that I am not strong enough. I am stubborn.
Often in this life, I try and be strong in myself. Sometimes it works. Sometimes I make it halfway.
Paul tells us to be strong in the Lord. When we trust in God's strength, we have more strength than we can imagine. We are not sufficient in ourselves, but in Christ... then we have real strength, and that strength has shown that it can defeat the biggest things in life that threaten us. Rules, authorities, cosmic powers -- these are things that my strength cannot approach, but in Christ, clothed in the whole armor of God, we have the ability to withstand evil.
So pray for God's strength, and do not rely upon your own. Then, and only then, can you stand firm.