Let me put it this way. In the OT fresh sin called for fresh blood. In the NT fresh sin is dealt with through the blood of Christ. Jesus doesn't have to hop back up on a cross to pay for your fresh sin. His blood is everlasting. He has an eternal priesthood and is your High Priest. He isn't like the priest of old, paying for His sins and then the sins of the people again and again. He did it once, and for all. Read Hebrews 10. So any new and fresh sin is washed in the blood of Christ thereby making Jesus' payment/sacrifice for sin take care of all sin (I.e, past, present and future).
I agree that the blood abides forever to forgive sin. But that doesn't address where is it written that a sin is forgiven before it is committed. Willingness to forgive something that hasn't happened yet is not the same thing as something being forgiven before it happens (as you folks teach).