That is because you are not uninstalling the regulator correctly. DO NOT use the bonnet as leverage, you need to use the regulator body. If you need to have the regulator removed and you have never done it before, I would highly recommend calling a proshop or field to see if they can do it for you. The $5-$10 charge will save you a lot of headache and possibly the cost of a new regulator.
If you heard grinding, that means there are most likely metal shavings that need to be cleaned out of your regulator now. You can watch this video after you degas your bottle and work on it: http://www.youtube.com/watch?v=94odI...UpCQA&index=11
The video will take you through the entire process of disassembling, inspecting, cleaning/relubing, and putting the regulator back together again.
Make sure to get all of the metal shavings out and verify there are at least 5 threads left on the reg body for the bonnet to screw onto. If so, you can safely use the regulator still.