“Be certain, one does not toil over a meal and misses it” (Ketubot 10).
We should interpret that since one can only work Lo Lishma [not for Her sake], since his nature is the will to receive for himself, if one dedicates much time and effort over the intention Lo Lishma, in the end he will wonder what will he get out of all the work that he had done throughout his life. If the intention is not Lishma [for Her sake], then it will all go to waste, since Lo Lishma is a lie, and a lie can exist only in this world. Conversely, in the world of truth there is no room for lies.
It follows that all the efforts he has given throughout his life for Torah and work, who will take it, since there is no room for this in the world of truth, and there is a rule that one does not toil over a meal and misses it.
According to the above, it follows that he will lose at once all the efforts he had made in this world, for the moment one must go to the world of truth, he leaves all his toil in this world. It follows that this calculation causes him to repent in order to correct all his work so it is Lishma, since he does not want his work in this world to be in vain.
Therefore, the advice is that if a person sees that he still cannot work Lishma, he should increase his actions in Lo Lishma, since when he sees that he has done many actions in Lo Lishma, he will have no other choice but to repent and work Lishma, or his entire work will be in vain.
The rule is that a person does not toil over a meal and misses it. Hence, if one has done many actions in Lo Lishma, he will not want to lose all his trouble, so he will need to correct all his work so it enters the Kedusha.
But one who works Lo Lishma and did not do many works, meaning he did not dedicate much time to the Torah and work in Lo Lishma, he will not have such a need to repent, since he will not have that many actions to lose. For this reason, we must try to do many good deeds even in Lo Lishma because this is the reason he will have a need to repent and work Lishma.