Starting the weekend in fourth, the Shaymen are now winless since August after going down 3-0 in Berkshire.
The game was as good as over inside 36 minutes as Josh Kelly scored twice and Adrian Clifton helped himself to the other.
Fullarton saw his team slip to ninth and wants quick improvement to prevent their good start going flat.
“We were below par and we got what we deserved on that day,” he said.
“We gave away some cheap goals and although we had some good moments you don’t win games at this level if you hand teams goals on a plate.
“There was nothing unlucky about this result and we’re very frustrated by a difficult afternoon.
“If you play like we did in the first half then you deserve to come away with absolutely nothing and that’s exactly what happened.
“We need to get back to winning ways as quickly as we possibly can.”