@soprano
If I were you, I'd go back and retake the one you got your highest score in. By continuing with other tests, you are probably forgetting a lot of what you previously learned.
Focus on two per window. Take one at the beginning of the window and the other towards the end. This way you will get the test back that you took at the beginning of the window and you will know if you need to retake at the beginning of next window while everything is fresh.
Try focusing on audit and FAR for now until you pass one and then loop in either REG or BEC. After passing one, you'll gain some momentum. You might also just be getting tests that are big on your weaker areas. You might have passed my audit test and I might have passed yours, but due to the luck of the draw, we got allocated tests that we were overall weak in.
Keep your head up. You can do it. And remember, your 18 months hasn't started yet!