Hearts ended a five-match losing run with goals from Ryan Stevenson and John Sutton, who was a constant threat along with strike partner Michael Ngoo throughout the game.
After watching his side miss the chance to leap into third in the SPL, a disappointed Lomas said: "We have let ourselves down and let down the fans who have come down to support us.
"We were under no illusion what was going to happen. We saw the second half against Motherwell when they pretty much went direct every time up to big Ngoo and Sutton and they caused us problems.
"But it wasn't just them two, it was all over the park. When you don't win your individual battles, you come unstuck."