How should I understand Kant' Formula of Universal Law?

The Formula of Universal Law (FUL) states: “One should act only in accordance with that maxim through which he can at the same time will that it become a universal law”. A maxim is a statement of mentioning an act a person wants to perform and the purpose for which the agent wants to perform it. For instance, one's maxim can be "I will make a false promise to return borrowed money in order to enrich myself". The FUL gives a necessary condition which a maxim must satisfy if acting on it is morally permissible - in other words, if a maxim violates the FUL, one is morally forbidden from performing the action for the purpose specified by the maxim. But this does not mean that the act itself is morally forbidden - only that performing it with the intention specified by the maxim is. Further, taken on its own FUL does not imply that if a maxim does not violate the FUL, then acting upon that maxim is permissible (in other words, it does not give sufficient conditions for an act being moral). It may be the case that a maxim is in accordance with the FUL, but violates some other moral rule. To rule this out, one would have to believe, as Kant did, that there are no moral laws which are not derived from the FUL.
The maxims which violate the FUL are those for which it is impossible to will that they become universal law. By "will", Kant essentially means "want"; by "becoming universal law", he means that the maxim in question is followed by everyone. But the difficult question is: when is it impossible to will that a maxim became universal law? On one interpretation of Kant, doing so is impossible under the following condition: if the maxim became a universal law, the action proposed by it would be impossible to perform. For example, If everyone was ready to make false promises whenever it suited them, there could be no promising: the practice of promising would be extinguished by constant violations. The problem is that many wrong actions are not of this nature. The maxim "I will break into a rich person's house to obtain some paintings" would pass the FUL test; If everyone lived by this maxim, it would still be logically possible to break into a house. This motivates a different interpretation, on which "impossibility" means that if the maxim was universalized, the purpose could not be attained by the proposed action. If everyone violated promises, statements like "I will give you the money back" would be ignored, and thus could not help the speaker enrich himself. In a world of constant art-robberies, people would not keep paintings in their houses: the robber might break in, but wouldn't achieve his purpose.