Hi there! The way I see it, the break-up started with episode 2.15. Brian freaked over the whole Justin/Michael/comic/don't-need-me-anymore thing, then over the intensity of his reaction, so while apologized and wanted everything to be ok again, he also wanted to take a step back - hence the not celebrating Justin's birthday desaster. And maybe normally that would have worked out somehow, but Brian pissing all over his work was a pretty hard blow for Justin, especially considering that Brian's appreciation of Justin's art used to be what he made least of a secret out of. So destroying that destroyed, or at least seriously damaged, Justin's faith in Brian's feelings for him altogether and he was in no condition to deal with the birthday disappointment on top of it.
The rest, I believe, is mostly lack of or miscommunication, Brian's fear of opening up when he's sure Justin's going to leave anyway and lack of confidence to stand up for himself, probably related to the bashing. As for the picnic - Brian had spent the last few hours at the hospital, remembering the time Justin was there, so the word romantic might have jolted him to the end of their last romantic night, causing a more extreme reaction than he might have had normally.
And I don't think Justin wanted romance, at least not in the flowers and candy sense, he wanted reassurance, something that was clear enough that he could be sure he wasn't just seeing what he wanted to see and banish all his newly acquired doubts.