Democrats let the Republicans dictate the message. Republicans were able to say "Obama bad" despite lower employment and gas prices, the Democrats did nothing of that, it was "despite" their efforts.
The Democrats were afraid to run on their record of more people uninsured, health care costs going down, better employment numbers and lower gas prices. Midterms are "all about that BASE" and rarely do independents vote. The Republicans were more motivated like the Democrats were in 2006. I doubt the Republicans will get close to the presidency in 2016.
I suggest that the more people uninsured was not making those whose healthcare premiums increased to pay for them interested in voting for the Democrats that forced the ACA through Congress. Health care costs are not going down. You sound like a Democrat. The healthcare rate of increase is less this year than it was last year. Gas prices are lower in spite of the best efforts of Obama to keep them high. The UE rate is down to almost as low as Bush's highest UE rate in eight years.
Not a lot for the Democrats to brag on that could not be immediately debunked, and I don't have a guess for what will happen in 2016.