Hi Totallyascaredycat and welcome to the forum

you are in the best company here when it comes to being terrified of dental procedures.
First of all, I doubt that you were lacking dental hygiene in the past years due to being dumb and young. I would rather say that you didn't know better and now you do. It's great that you are ready to address your problems now and are on your way to save all other teeth and keep them healthy long term. Give yourself a pat on the shoulder for having made it to the dentist at all. By the way, a good dental cleaning can make a lot of yellow ugly teeth looking nice again so this might be also your case.
I understand that an extraction sounds very scary, but you might be surprised to hear that extractions are often the quickest procedures. From my experience the waiting for the numbing to take effect takes the most time. You will be numb so there won't be any pain and your dentist will hopefully talk you through and try to put you at ease.
Your fear of needles and your gag reflex would be good topics to get discussed with your dentist. If you worry about gagging, let him explain why it won't happen this time (probably because he just won't touch that part of your mouth that provokes gagging). Making a mold cannot be compared to an extraction when it comes to gagging. Which tooth is it that needs to come out?
Scared of needles. This one is my favorite, because I am too (and the most people and even the ones that don't consider themselves as scared of the dentist). Tell your dentist about this one and tell him you do not want to see anything. He will take then work below your sightline. I also always look up up up as much as I can not to see anything. Be aware of the fact, that being numbed is no comparison to a needle anywhere else. It you think of blood tests or getting shots, it's not like it feels. Your dentist will use a numbing gel so that you wont feel the needle going in. You won't even notice a pressure, you won't notice anything. The second step is that your dentist will numb you really really slowly. This is the key for you not to feel anything. Believe me or not, but this is how a shot feels when my dentist is doing it: He takes a tool (I do not see which one because I look up), the holds the tool in my mouth (I can feel it resting on my lips) and we stay like this for ages till he announces that it's done. It's the most boring part of any treatment, you are just being there and waiting. So it's not just about 'not having pain', it's about 'not noticing anything whatsoever'. It might be a good idea to ask your dentist how good he is with needles and if he can make it painless.
Here is a link that you might find helpful:
https://www.dentalfearcentral.org/fears/needle-phobia/
If you, after talking to your dentist still feel absolutely unable to get through the procedure, you might consider finding a practice that offers nitrous or even sedation.
All the best wishes and let us know how you got on