As for the note regarding "今さら意識してしまったので", what he's saying is more along the lines of "but now I keep thinking about it" In a sort of 'it's different now that we're dating' meaning. I suppose the easiest way of translating that shortly might be 'but we weren't dating then' or 'but we're dating now.'
意識してしまった means to have become aware of something. So now that they're dating even the most minor and mundane of his flaws worry him. To say it simply, he's self-conscious now.
Hope that helped clear things up and that I wasn't putting this in the wrong place.
If you want a bit more of a breakdown of the grammar being used, してしまった is usually used when it's a result that wasn't planned for. 'I slipped' 'I broke it' 'I screwed up' sort of scenarios would all fall under してしまった. He's trying not to think about all the ways he could mess up their relationship but his mind keeps going back to it despite himself.