Sorry to hear about your struggle. Picking yourself up after a setback like that is tough! I have coached many students through this exact situation, so you are not alone.

If you do more of the same, don't expect different results. You need to fundamentally change what you are doing. I recommend a restart using two paradigms:

(1) Simulate GMAT-like conditions as closely as possible, and
(2) Study using depth instead of breadth.

To simulate GMAT-like conditions you need to be taking a CAT that you have never seen before. My guess is that you got your 680 by taking GMAT Prep more than 2 times. GMAT Prep is really only useful for the frist two tests. After that, the data are erroneous. You need to find a source of new CAT's to work from. I recommend them in this order:

a. ManhattanGMAT (NB, I'm biased)
b. 800Score.com

To study with more depth, make sure that you are spending at least two minutes reviewing material for every 1 minute that you spend doing material. You need to find a new source of GMAT problems. Why don't you use the BTG problem archives for those. Keep a record of what you have learned from EVERY problem. Review these notes in their entirety weekly. When you get quick, these review sessions should take no longer than 10 minutes total.

Remember, if you want different results, make sure you change your approach.