The authors only regret that they should have come to an end. But if we have helped you master a foreign tongue with some success, our task has been worth its while.
This is the last paragaph of the epilogue of one of my english textbook. I am not able to understand why "should have come to an end" is used. I know that for example "I should have gone to bed early" means "I didn't go to bed early and now I'm tired". So my sentence means that the authors did not come to an end which is nonsense. Can you explain to me this particular usage in my sentence above.