The Bees suffered an early setback through Michael Rankine's opener, although David Stephens may have got the final touch to turn the ball into his own goal, but strikes from Andy Yiadom and Jake Hyde sealed the win that moved them two points clear of the drop zone and into 20th place in the table.
"It's another blow for us," Mills said.
"It's another game we've lost, we're not winning football matches and it's another game we've lost to an opponent lower than us.
"We've got a throw-in in front of our dugout and within seconds it's in the back of my net. It's as simple as that.
"You can't be giving goals away like that and we've been giving goals away for quite a few weeks now.
"We've not scored a lot of goals in abundance and we know that's a problem, but when you're conceding goals like we are doing at this time it's going to make it very difficult for ourselves.
"It's got to be so much better and we can't give goals away at this level of football and expect to get away with it.
"We're getting punished in a big way at the moment and deservedly so because you can't be sloppy like that."