Hey There...Yes it is deep and you may need to get Root canal treatment done to save this little tooth. The drilling is not painful. Dentist will give you anaesthesia to make that tooth numb and so, you will not feel any pain.
Next Steps
You should seek Dental consultation before the pain gets bad. Remember, delaying any treatment only increase the expenses related to that treatment. Sooner the better
Health Tips
Avoid eating or drinking anything too cold from that side