As far as I understand, if we accept utilitarianism ethics, then the right thing to do is to maximize the happiness of everyone, including all sentient beings. But this does not leave any space for any personal project that does not contribute the common good.
For example, assume that I am very interested in writing poetry, but am not interested in publishing or unable to publish anything. Then my time spent on poetry does not help anyone other giving me some satisfaction. According to utilitarianism, I should throw away my notebooks and do something like volunteering for a charity.
But this seems to be really difficult even for the noblest of us. So for utilitarians, the justification allowing some personal project seems to be that it is impossible for humans to get rid of self-interest.
Are there any better answers to this question?