Have you received interviews? If so, have to requested any feedback as to why you were rejected? If you haven't received interviews have you also requested feedback on this?
Have you sought advice from tutors/mentors as to why you might have been unsuccessful? Perhaps have you looked at the university websites to see what they look for outside of the academic requirements in terms of work experience, manual dexterirty proof etc.? How were your scores in admissions tests like UCAT etc.?
I'm always a believer that no application is perfect and there are things that can always be improved upon. That being said you have applied twice without succes and unless you think you can make a significant improvement on your application for another try, you may need to consider carefully whether this is best use of your time and resources. You could of course apply for an alternative programme alongside your Dentistry options, for example Pharmacy or Optometry and if you got the equivalent of at least a 2:1 or 1st, could consider applying to study Dentistry as a graduate?
The flipside is if you were a qualified Pharmacist or Optometrist you would also be able to work as a locum (if you have the time) during your studies to supplement your income and they're both excellent careers with excellent prospects.
Don't rush into making a decision, you still have a while yet. Indeed if you wanted to apply and study Dentistry abroad that is a possibility. You could even perhaps start abroad and reapply in the UK? There's the Irish Schools as well which are super competitive but they're an option.
Maybe sit down with a careers adviser or your tutor and discuss your options and see what works best for you. Neither of the above options means giving up on being a Dentist and tbf you only come to that point if you personally decide you no longer want to pursue it.
What's required here is a an assessment of what you've done right and wrong up to this point, how you can improve and perhaps place the strongest possible application to give this a further shot.
Good luck.