Is it permissible to supplicate against oneself that Allah not bestow a bounty if that which is customary of it is great and difficult to fulfill?

You should not supplicate for the non-occurrence of something that appears to be good. However, it is not forbidden to supplicate that the matter upon which you made a vow contingent does not occur, as long as there is no sin, no severing of kinship ties, and no aggression in supplicating for its non-occurrence.