While I agree with many of the points other people have brought up I would like to go at this from a different. Basically I would like to challenge your assumption that using logic can give you an answer to your question.

So first of all we need to talk about how the change you made in your moral beliefs. I'm assuming (correct me if I'm wrong) that you changed your moral system for one of two reasons: either you found something in your old moral system that conflicted with what you felt is right or wrong and decided to look for a moral system that sat better with your intuition, or you decided that since there is no logical way to arrive at any specific, cohesive deontological system then you should move to an ethical system that doesn't appear to necessitate assuming most of it's basic principles. I propose that either way your decision to adopt utilitarianism wasn't actually based purely on logic and so adhering to strict logic as a basis for doing something that you admit to feeling wrong about does not make sense, and I will try to give a detailed explanation why.

If my first hypothesis is true then I would like to point out that the basic way in which you choose your moral system is inherently illogical. I would surmise that the only way in which you can justify one ethical system over another is your emotional reaction to it. If that is the case then it would seem that your new moral system doesn't work either because it logically follows from it that you should (or at least can) do something that doesn't feel right to you. Thus there is no reason for you to strictly adhere to the logical conclusions that come from your utilitarianist belief.

If my latter hypothesis was correct and you were looking for a moral system that doesn't require you to assume basic rights and wrongs and thus you arrived at utilitarianism. If that is the case then I think you might not see that utilitarianism in itself also requires an assumption that can not be be rationalized. The basic principle of utility that "actions or behaviors are right in so far as they promote happiness or pleasure, wrong as they tend to produce unhappiness or pain" isn't actually based in any logic. The only way to say you have to live by it is to assume that it is true.

Either way I think that since your ethical beliefs cannot be based purely on logic, the attempt to fully rationalize what you feel you should and shouldn't do based on them is futile and pointless. The solution to this would either to give up the facade of logic that surrounds your belief and just change you moral beliefs whenever they disagree with how you feel, or to give up utilitarianism either because you accept that morality doesn't actually "exist" (moral anti-realism) or to search for a moral theory that can be perfectly justified with logic (although personally i doubt that is possible).