Hardest for me is definitely talking about attachment (and transference) to him and also how I used to feel about my marriage counselor (was still feeling that way about MC when I started seeing current T). And things tied into that, like admitting the other day that I'd watched a streaming video of him online. Those are the topics that make me dissolve into tears and have trouble looking at him.
Like a couple other posters, even though he's male (and I'm female), I seem to have no trouble talking about sex/intimacy topics with him. It's even easier than with ex-T, who's female, though I had negative maternal transference for her, so maybe it felt like talking to my mother...(she's also my mom's age).