1) It is not recommended to use harsh measures to make the child stop feeding such as applying medicines on the breast.
Breastfeeding a child past two years is makrooh (not haram). My humble suggestion would be to start with tawassul with the Ahlulbayt (as) for help. Secondly you can consult your doctor or lactation consultant on ways to stop.
2) If you feel fasting is affecting your health then yes you can skip. Otherwise missing a fast without a genuine reason is not permissible
As for fast missed during breastfeeding, you have to pay the fidya for every missed fast (750 grams of food to a poor person) + qadha and if you did not pay the qadha before the next month of Ramadan then you have to pay kaffara e takhir which is 750 grams of food to a poor person for every missed fast.