When you go to the dentist the first thing she will do is not to fix that cavity, but an inspection of your teeth. This will give you a chance to meet and talk to the dentist before anything gets serious and you can ask all the questions you want.
The truth about the pain is, filling cavities used to be extremely painful in the long gone days before anaesthetics were invented. These days you get a jab, the part of your mouth with the sick tooth will go numb and you will not feel a thing during the treatment. Even after the anaesthetic wears off there won't be any pain as there is only pain during the cleaning of the cavity but it stops right after the tools are removed from the tooth, this is where teeth are different to flesh wounds.
On the other hand, if you wait too long you will be in for a lot of pain. From your description I would think that your teeth are overall rather healthy. A healthy tooth can be very sensitive even with a small hole that is not very deep. This means there is a lot of potential for the pain to become worse. At some point it will become permanent. This can happen suddenly. You may wake up in the middle of the night and will be in pure pain, as if you had a glowing hot iron in your mouth and you can't get rid of it. Also there will be no one close by to give you any real help (like a dentist could).
So, if you think going to the dentist is scary, not going is a lot scarier.
How do I know all this? From experience. From my mothers side I inherited teeth that are prone to cavities so I had regular treatments starting with my milk teeth. The only dentist in the area believed in treating children without anaesthetics so they would remember the pain and look better after their teeth. This was forty years ago and any dentist with this kind of attitude today would not have their license for very long (or so I hope). This left me scared of dentists too and as an adult I didn't go for a couple of years, until one of my teeth became so sore as I described above. The pain was a lot worse than getting treated even without anaesthetics and there was no end to it.
Today I am more scared of not going. Because of the condition of my teeth there was a time where I have been more frequently to the dentist than to the hairdresser and these days, having been to sane dentists ever since, I am not scared of going at all.