I don't think you're SOL! I took the GMAT 2 yrs ago (using a self-guided Manhattan course) and I followed their advice taking practice tests every few weeks and got a 540 on my first one. Personally, I was really frustrated testing myself without having covered a lot of the material yet. I studied for about 4 months and got my official score up to a 640 (still not happy which is why I'm taking it again now).
My new strategy 2 years later has been to go through all of the materials FIRST, then start taking the tests. I took my first practice this weekend and got a 620 - still not happy with it, but at least it's a better basis to start from, knowing that I have gone through all of the material.
I guess the one upside to taking consistent tests without having learned all of the material is that you will be used to the time constraints which I definitely struggled with this weekend.
Don't give up so easily! You have a lot of material to get through and your score will improve as you learn more. Good luck!