This does not look like damage that I would expect from tightening too tight. It looks like it has been cut by working on a damaged surface or the wrong type of surface. Sealing at the end like this is not where I would expect to find an O-ring but rather a flat washer if the seal is to be on the flat end. I think you need to step back and make sure you are putting the right combination of fittings together. This is not making a normal seal.