Hi from Winnipeg! Somewhere along the way, your tooth has suffered some form of trauma. This causes the tooth to discolor since more dentin builds up in the tooth and it is not a white color and so it shows. An x ray would show this . The simplest way for you to whiten that tooth is...
The initial photo after whitening appears that the teeth are dried out which make them appear whiter. When they rehydrate as they should, they do look alittle darker or more yellow. So the first picture is deceptive. The final photo does look as though you did get some good whitening effect...
The major problem with whitening and perio disease is sensitivity where the roots are exposed, ie where you have recession. Therefore, I would recommend custom trays made by your dentist that just cover the teeth. If the peroxide whitening agent is not on the roots ,you will get much less, if...