Yes, we have not been successful at winning the special elections. That being said, we were not supposed to. When you normally lose by 20 points and now you only lose by 5 points, it provides a realistic opportunity (and frankly expectation) to win in purple districts where the past few cycles have been much much closer.
As stated and I will sadly not dispute, the democrats have overall lacked a backbone. Yes we have Clinton, Biden, Sanders, and Warren - but they are pretty much the only ones willing to bark as loud as many many republicans. Now just because you are the loudest it does not make you correct, but that is the way to win elections in the United Sates.
Because of this, I do agree with your assessment that we are unlikely to take a super majority in congress after the '18 election. That's not our objective though; we want to take a super majority from the 2020 election and 2018 will serve as a building block to that. Our goal is to ensure Trump or Pence is a lame duck president after next year and we prevent him from doing our country (or at least our agenda) harm. Then in 2021 we can achieve (by force if we have to but I think there will be some republican votes like there was in the Affordable Care Act) single payer medicare for all health care. That should and will be our top priority.