Christian Has Deep Doubts About Their Salvation

How can I know that I'm saved when I don't feel saved? I know that many people struggle with doubts about this – it's a problem I've had throughout my Christian life. I've talked to many pastors over the years, and nearly all of them have told me the same thing. They said that if I've asked God to save me, then I'm saved. I also know what the Bible says about this – that the Lord will save us if we simply call upon Him. But that knowledge doesn't always help. A few nights ago it got so bad that I was completely convinced that I am not saved and never will be. It was more than just a doubt, more than just a feeling. It was debilitating. I was beyond functioning. I can't go on living with this kind of despair. Can you help me?