I think the issue lies in the practice test environment. Like veni mentioned, it's unlikely for you to get an 18 if you were consistently getting 20+ scores on bootcamp. During your studying, you should try to replicate the test environment as much as you can (laminated sheets of paper, expo markers, taking gen chem, o chem, and bio back to back, etc.).
As far as applying to dental school goes, you definitely can apply. The average DAT score for accepted students into dental school is roughly 20. If you really believe you can't improve your score with a retake, apply this cycle and apply broadly.