People put WAY too much stock into the diagnostic. The point of the diagnostic is to see what you need to work on. You aren't expected to do well. Sure, there are people who get 30+ on the diagnostic, but who cares? You just use these practice tests to see what you need to work on. I pretty much stuck exclusively with aamc after about 2 kaplans. I'm retaking them now, but I am 3+ months removed from a few of them. It isn't like I remember 22: "a" and 23: "c". In fact, retaking them kind of caused me to be worse at first. I would try to reflect on that moment and misremember, so I just rethink every question and go through the thought process now.
I had a 20 diagnostic (or 21..can't remember). My last practice tests were a 33 and a 32. The mistakes I made weren't on hard questions either. I just wasn't mentally "there". (My good friend died the day before) So, yea...plenty of time and room for improvement. Just keep testing and finding out what you're screwing and why.