"The game got away from us in the opening 20 minutes and you can't give any team two soft tries but that is what we did," said the former Irish international.
"The charge down was one which they didn't have to work for and then they scored off a set piece.
"When you gift them 12 points in the first half then it is always going to be an uphill struggle, and we did that last week as well when they got into an early lead."
Scarlets came from 17-3 down to level the game at 20-20 early in the second half only to concede ten unanswered points as Gareth Steenson notched his third penalty of the afternoon and Exeter skipper James Scaysbrook touched down late on under the posts to condemn Scarlets to a fourth straight defeat in the competition.
Earlier, scores from Ken Owens and Scot Williams had dragged Scarlets back into the game, with Thomas's conversion drawing the Welsh outfit level shortly after the restart.
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