Why am I struggling with intrusive thoughts?

Hello, I am Christian and I am a teenager and lately for some reason I have been struggling with atheistic thoughts and doubting God's existence. They just come out of no where in the most random times when I am the most vulnerable and I also have a problem with blasphemous thoughts and I always think that I am committing the unforgivable sin. After these thoughts occur, I feel strongly guilty and I feel like I have been rejected by God and I feel miserable. I know these thoughts are of Satan and he is using this to take away the joy of being a Christian. I will never reject God. And I just want to ask that am I too far from God for forgiveness? I thought God said that He will never forsake us but I always feel like that and I feel so miserable. I need some really good advice