I think there are only two likely scenarios that will happen for the Democrats in 2018. The Republicans will be gaining Missouri, North Dakota, and Indiana for sure without a shadow of a doubt. The first scenario, which seems pretty likely, is that Trump is moderately unpopular to flat out unpopular and it damages the Republicans in western states. I think if he is unpopular then Democrats will surely gain Nevada and Arizona putting the GOP at a net gain of 1 seat with a 53-47 GOP majority. Should Trump be popular the GOP will gain Ohio, Wisconsin, Florida, Montana, and Pennsylvania at a net gain of 9 seats leaving the composition to be 61-39 GOP majority. Putting my biases for the Republicans aside, strictly from an analytical standpoint I find it hard to believe the Democrats have a chance at gaining any seats. The numbers are simply not on their side as they control 23 seats and Trump had won ten of their home states and came close to winning 3 of them. There simply is no pathway for them to create a majority in the Senate.
Relax.