Dear Boss Lady,
I regret to inform you that I'm leaving (your company). It is with reluctance that I'm submitting this letter. I have enjoyed working with everyone and have learned a great deal, and my departure in no way reflects badly on the company. Although my time with (your company) has been, on the whole, satisfying and productive, for quite a while now I have become less and less satisfied with the work situation. The split schedule, work load, and the living situation are making it increasingly difficult for me to continue here. Please be informed that I am resigning as of ８月３１日. I'll be happy to help train a replacement before I go.
(Taken straight from the first Google result for "resignation template")