Acid, fruit juice and brushing.

Citric acid in fruit juice softens the surface of teeth, making them vulnerable to damage if you brush immediately.What to do?

1. Allow time : The teeth reacquire  minerals from saliva over a period and become hard again.
2. Use less aggressive  toothpaste : Use toothpaste for sensitive teeth.

tl;dr? Executive summary : If you have fruit juice with breakfast adjust your routine so that you brush a little later than normal. Use Morrisons Sensitive*.

* Or any generic sensitivity toothpaste.