The only hazard is if a cell or group of cells is closed off from the blood supply. A tourniquet on your arm will after a few minutes turn your hand blue and then black and then gangrenous. That is what the refusal to forgive does to us.
This is pictured for us in the words of Jesus. "This is my blood of the new covenant." Or as Paul explained, we are made right by his blood (Romans 5:9). Jesus's blood on the cross made visible for all to see his continual cleansing of every person's sin and the life-giving oxygen that we need. And that goes on continually without any good works on our part.
But what if we close ourselves off from that cleansing and life-giving stream? We can refuse the continual taking away of our sin and insist on making ourselves good, which of course does not work. We can also refuse to forgive others. It is easy to see in a neighbour how quickly that festers and forms an abscess where poison gathers and becomes very ugly. It is not so easy to see it in ourselves.
As you were taught, you have eternal security by the eternal bloodstream of the love of God. Having been "ransomed, healed, restored, forgiven" (H.F.Lyte's hymn Praise, my soul, the King of heaven), it shouldn't be hard to let that love to flow out in forgiving others, however badly they have treated us (see Matthew 18:22-35).
Forgiving others does not mean being a doormat. We don't have to invite a rapist to our home, or let a pervert take our children out to the woods, or deal with a dishonest business person. Nor does a woman have to put up with a man who refuses to go to AA for treatment, beats her up, or abuses the children. But she can forgive and be very firm.
(I got the illustration of closing off the bloodstream from Rev. Dr.
David Ward in his Valentine's Day sermon in St. Paul's Anglican Church,
Kingston, Ontario on February 14, 1999)