Awwww, the biggest comfort is that it will be soooooo much better afterwards. Don't worry think about this. From now on it will only get better no matter what the diagnosis is. Diagnosis is the beginning of healing even if it sounds scary, it only states what is in the past and behind you. If it sounds bad or scary think - it is the past. And whatever will follow form there will be relieving.
Don't let the doc judge you - if they do change practice.
Don't self shame, blame yourself - you have already been through enough. Past cant be changed anyway.
If you are scared ever about pain - no dental procedure will ever be as painful as your tooth ace.
For the doctors this is their job, their profession, whatever they find they are trained to treat you so you should be in good hands. They have seen and dealt with worse then your situation and might actually like it if you are a bit more challenging patient then someone who goes in for teeth whitening.
As for tomorrow they can give you painkiller on injection, they will do Xrays, cleaning and they can treat the tooth that is hurting
You will be fine, the pain the fear will be over after the appointment