Our sufficiency is in word, to believe completely and have no doubt,
The bible says in philippians 4:13 "I can do all things through Christ which strengtheneth me."
Our strenght is from the Lord.
There are times when God knows we need a "thorn," a challenge, to keep us dependent upon Him. If we are wise, we will rejoice in it, that the strength of Christ might empower us and that God might be glorified. But please notice that Paul received an answer to his prayer. He did not go through life wondering why God had not heard and answered his need. The relationship Paul had with God allowed him to fellowship with God as his "Abba, Father" (a term of close endearment such as "daddy"). Paul held nothing back from God, and God held nothing back from Paul!

Claiming the sufficiency of Christ, and realizing our sufficiency in Christ, is the absolute key to being victorious in our battles. Yes, we must be free from our attitude of self-sufficiency. We must have the might of Christ in our challenges, and in our weaknesses allow Christ to be our strength!
Christians be Wise!
Thanks @olawalium