It will be interesting to see how you do on the real DAT. The scores you get on these different programs use an estimated scaled score and that can be adjusted internally within the program. To say DAT Achiever is hard just because you got a 14 or 15 may not mean much unless you actually missed that many questions. I found that topscore reflected the scaled score more accurately than datachiever. You have to compare the number you miss with the scaled score results rather than just the scaled score you receive from the testing program. In otherwords, the scaled score is adjustable. The best thing to do is to use the program to see where you are missing topics and then find out if those are fair topics to test.
If for example, you miss 10 questions on datachiever and those questions are not fair topics to test (you can see if they are covered in the scope from the ADA) then you are wasting your time and getting depressed over questions or studying details that are NOT tested.
Study the right topics from your college text books and do some sample tests. That's the best way.