My DAT is 3 weeks away, on the 26th. I've been studying for an average of 4 hours a day since about May 20th. The days I have off of work I generally study for more like 8 hours. And I've been using Barron's and Destroyer, with my old textbooks for all my studying.
Honestly, I did really well on all the Destroyer OChem problems, and have scored very high on the practice exams I've taken for this section. (Around 22-23) But GChem and Bio were another story.
I started off in the same boat as you, missing about 40-50% of the questions. But believe me, the more you study what you missed, the more you will retain, and the better you will score.
I study by taking notes of everything I missed, or didn't know, and saying it out loud as I write it down. I do about 50-100 destroyer problems a day. At the end of each day, I go over all the notes I took for that day. This method has really helped with my retention.
Now, though not great, I am getting about 70-80% of the destroyer problems right, and have brought up my practice exam scores in these areas. (From 16-17 to 18-20s) I know this method takes alot of time, but you still have plenty of time to work with. I hope that by the time the test comes around I will be scoring 22s in each section on practice exams, and from what I hear, the real DAT is slightly easier than the Destroyer. Good luck with your studies, I know you'll do well!