Hello!
Sorry to hear about your current position and that it has put you off nursing!
Its quite normal to help with training of trainee nurses once you're qualified, and actually its better that you are involved in this as you are in a better position to make sure he/she is carrying out all the infection control procedures etc in the same way as you. Remember, if he/she makes a mistake, they are training, but as you are qualified, it is you responsibility to protect your registration.
Have you approached your dentist about how you feel regarding the responsibility of 'acting practice manager'? Check your contract. While its useful to have an understanding of admin, it is a separate role, and if you are not happy to carry out the extra work, discuss options with your employer. Worst case scenario is he doesn't make any changes and you move on to work at a practice you are happy with.
Dental nursing is not a dead end job at all. There are additional skills that you can build on such as Oral Health Promotion, Fluoride Varnish application, sedation nursing etc
Practice management is a path that some nurses go on to pursue if interested.
I have gone on to study dental hygiene and am loving it!
Try not to let one bad experience ruin it for you.
If becoming a paramedic is your dream, you could study an access course for that via distance learning while you are working. Then once you are ready, you can apply for that. You are still young and the world is your oyster.
Hope that helps