Well technically he did win, he strengthened his control of the Senate.
Nope he got fairly hammered. Theres technicalities with the Senate results and the GOP are worried about the election in 2020.
In quite a few safe red states that Democrats have got no where near competing in past decades, they narrowed the GOPs lead to an insignificant amount (in some cases nearing the levels of the being able to demand a re count)
When you look at the different factors in play Republicans are worried with where they stand
1. Voter suppression
2. Gerry meandering
3. Safe red seats having their margins drastically reduced
4. Traditionally Republican voters always turn out for elections whereas Democrats don't however in this case there was a larger Democrat voter turnout
5. Presidential election years always get a bigger voter turn out than mid terms and the next one in 2020 is when a lot of Republicans senate seats are up for election (only a third of senate seats at a time are up for elections)
So contrary to what trump may say, the results aren't looking good for them