"Let us hold fast the profession of our faith without wavering; (for he is faithful that promised;)" (Hebrews 10: 23)
When we read passages like "hold fast", we may be tempted to thinking of faith as a struggle, or something that we must fight through strenuous effort.
Jesus has given us His faith to believe on Him:
"8For by grace are ye saved through faith; and that not of yourselves: it is the gift of God: 9Not of works, lest any man should boast. " (Ephesians 2: 8-9)
"20I am crucified with Christ: nevertheless I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me. 21I do not frustrate the grace of God: for if righteousness come by the law, then Christ is dead in vain." (Galatians 2: 20-21)
Paul writes to Timothy:
"Fight the good fight of faith, lay hold on eternal life, whereunto thou art also called, and hast professed a good profession before many witnesses." (1 Timothy 6: 12)
The better translation (although somewhat awkward for an English-speaking audience):
"Let the good fight of faith be fought."
Jesus defeated Satan, sin, and overcame the world at the Cross (John 16: 33).
We just need to believe that He finished everything when He cried out: "It Is Finished!" (John 19: 30)
God came through for us when we weren't even looking for Him, when we were lost, and had no idea that we were lost:
"But God commendeth his love toward us, in that, while we were yet sinners, Christ died for us." (Romans 5: 8)
Even when we waver and we want to trust ourselves, He is faithful:
"If we believe not, yet he abideth faithful: he cannot deny himself." (2 Timothy 2: 13)
Look to the Parable of the Loving Father (Luke 15) to see how faithful God our Father is, even when we as sons think that we can live on our own without Him.
God cannot lie, and He swore by Himself:
"18That by two immutable things, in which it was impossible for God to lie, we might have a strong consolation, who have fled for refuge to lay hold upon the hope set before us: 19Which hope we have as an anchor of the soul, both sure and stedfast, and which entereth into that within the veil; " (Hebrews 6:18-19)
He is faithful. When we understand this, we are growing from faith to faith (Romans 1: 17), and we can hold fast the profession of our faith in Him.
Don't focus on your believing. Focus on how faithful God is, and you will find yourself believing on Him!