My advice to you is to try to separate the emotional issues from the mathematical questions and you answer will become clear.
When addressing a refinance, weather you have lost or gained equity over the past 18 months is not where you should be focusing your energies. What has happened in the past is done. Going forward, ask yourself how long you intend on owning the home (either as your home or holding as a rental property). If it is likely you will want to sell within the next few years, then it may not make sense to buy down further equity (or pay PMI alternatively) and incur costs to refinance.
If it looks like you will be holding the place for a long time, this is an ideal time to to make adjustments to your long-term financial picture (perhaps design your morgage so it amortizes just as the kids are about to enter college). While it may mean some sacrifices now, it is unlikely that rates will remain this low for much longer. If this is your situation, I suggest that you consult with your mortgage broker (or a few of them) and have them lay out your options for you (30, 20, 15 yrs fixed; with PMI/without PMI;...), compare them side by side, and determine which option meets your needs while remaining within your budget. Depending on what the terms of your original loan are, your best option may be to stay pat, but this is still a good exercise to go through.
While I am sincerely sorry that you seem to have lost some of your original investment, I suggest you do not throw the baby out with the bathwater. You still have some of your original investment left and a wonderful opportunity to reduce your long-term holding costs.