When it comes to the Senate, Democrats managed to pick up the senate seat in Arizone. While they actually lost two Senate seats on net, the map concerning the Senate races was very unfavorable, and Democrats actually performed better than was to be expected.
Finally, when it comes to the state legislature, Democrats managed to capture several key battleground states where they now control the state Senate or state House of representatives:
Here more information: https://ballotpedia.org/State_legislative_battleground_chambers,_2018
Finally, one last point on the election in 2020. While there is usually a large incumbency advantage for the president, betting markets actually now slightly favor Democrats to pick up the presidency in 2020. According to 538, just mobilizing the Republican base like Trump does will not be sufficient, as the results in the midterms have shown.
A decent Democratic candidate should be able to win some of those key Midwest states (Pennsylvania, Wisconsin, and maybe Ohio) that Trump won and which gave him the electoral college.