I'm not the developer but allow me to answer. Take out the FAST option whenever there's a choice to avoid skipping over important text. Also, getting the different endings relies heavily on getting the butterfly mug from Emma. And getting that is tied to your very first interaction (push her only slightly to talk about how she feels about her dad's new girlfriend, comfort her, that sort of stuff).
Not sure if equally important but, also, choose Lizzy over dad, flirt heavily and choose telling the truth whenever the choice is available to you. There are two different endings that depend on whether you use the cup in the final act or you don't.
Happy endings hunt!