I'm quite surprised it hasn't done the job for you, but i'd definitely consider adding eggs to the equation for a week and seeing what happens. Just to make sure, you're using the "Sensodyne" Pronamel toothpaste right? And the "daily toothpaste" one, not the "whitening alternative". Other toothpaste brands just don't cut it i'm afraid.
Can you remember when you originally started using the toothpaste and whether it helped back then? This may be a problem which was partially helped initially but returned in abundance later.
If i could i'd definitely give that kefir yoghurt a go for a week and see if that helps. I would suggest adding it alongside the Greek yoghurt rather than having one over the other but make sure you have them at separate times of the day to maximise their benefits or if you have them with meals maybe one before and one after.
I would personally love to try either out but i'm lactose intolerant so it'll be more of an issue than a help in my case.
It may help if you state your diet so i can analyse whether there's any individual foods there which may be amplifying the problem. Be sure to include supplements and beverages as well!