Hello
@Sara100 - cracks such as you are describing can be a sign of a yeast infection, or vitamin deficiency (often Vit B), or anemia, just to mention a couple of things.
When your doctor diagnosed you did he also run a general panel of blood tests? If so, it might be worth asking if anything showed up there.
In the means time, a good lip balm could help no end, although I would be very careful of double dipping. If you apply with your fingertip, and need to add more, use a different finger, in case you begin to contaminate the balm with a yeasty thing.