It is not undemocratic.
You complain that Remain failed to convince people of the implications of leaving.
Clearly, the other side made a better case and won.

The customs union and single market played tenth fiddle to "taking back control," not giving the EU our money.....

If true, Remain allowed it to happen.
They had the same opportunity to put the case as did their opponents.
It was a fair trial and a verdict was reached.