That’s true. But also keep in mind that a sizable number (10, I think) of the 23 Democratic seats up for election in 2018 are in states that Trump won in 2016. From what I can see, only two GOP seats (Nevada and Arizona) are potentially vulnerable.
Historically, the party in the White House loses seats in the mid-term election.
In 1994, it was enough to put both the House and Senate back in control of the Republicans, for the first time in decades. It happened again (for the Democrats) in 2006, and again (for the Republicans) in 2010.
The map may appear to favor Republicans, but it may be the Democrats ready to crawl over broken glass to vote against Trump's agenda.
If Republicans aren't satisfied with Trump's progress, they may stay home.