I have been a christian since age 15. But have suffered from depression at various times. Sometimes it hurts so bad that I have grown so weary and even doubted my salvation. I am tired of doubting, I just want to believe but it is not in my power to do so. God must give me that gift.
God gave you faith to believe when you were 15.
He is the same God today.
It doesn't take 'great' faith to be at peace with God, but faith the size of a mustard seed ,in a "Great' God.
Stop looking at your own strength, and turn your eyes upon Jesus.
Cast your cares upon him, for he cares for you.
Look full in His wonderful face, and the things of earth will grow strangely dim in the light of His glory and grace.
I would recommend you so fill your eyes ,and heart with the word of God, so there is no room for Satan to make you doubt.
That is his method.
Recognize it for what is is, and turn from it to that gift of faith God has given you, then ,from faith to faith allow it to grow by the grace of our loving Savior, Jesus.
Father ,Help this your Son, to overcome, and grow in grace and knowledge of you, in Jesus name, amen